diff --git a/components/atoms/static_atoms.txt b/components/atoms/static_atoms.txt
index 8409639055f..2d0ef951067 100644
--- a/components/atoms/static_atoms.txt
+++ b/components/atoms/static_atoms.txt
@@ -1,3 +1,5 @@
+-moz-gtk-csd-titlebar-radius
+-moz-gtk-menu-radius
 DOMContentLoaded
 abort
 activate
diff --git a/components/script/dom/cssrule.rs b/components/script/dom/cssrule.rs
index a541ee411c7..853a9c9b643 100644
--- a/components/script/dom/cssrule.rs
+++ b/components/script/dom/cssrule.rs
@@ -105,6 +105,8 @@ impl CSSRule {
             },
             StyleCssRule::Page(_) => unreachable!(),
             StyleCssRule::Document(_) => unimplemented!(), // TODO
+            StyleCssRule::Layer(_) => unimplemented!(),    // TODO
+            StyleCssRule::ScrollTimeline(_) => unimplemented!(), // TODO
         }
     }
 
diff --git a/components/script/layout_dom/element.rs b/components/script/layout_dom/element.rs
index adbeba7e39d..ebd54df0c31 100644
--- a/components/script/layout_dom/element.rs
+++ b/components/script/layout_dom/element.rs
@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use crate::dom::attr::AttrHelpersForLayout;
 use crate::dom::bindings::inheritance::{
     CharacterDataTypeId, DocumentFragmentTypeId, ElementTypeId,
 };
@@ -264,6 +265,16 @@ impl<'dom, LayoutDataType: LayoutDataTrait> style::dom::TElement
         }
     }
 
+    #[inline(always)]
+    fn each_attr_name<F>(&self, mut callback: F)
+    where
+        F: FnMut(&style::LocalName),
+    {
+        for attr in self.element.attrs() {
+            callback(style::values::GenericAtomIdent::cast(attr.local_name()))
+        }
+    }
+
     fn has_dirty_descendants(&self) -> bool {
         unsafe {
             self.as_node()
diff --git a/components/script/stylesheet_loader.rs b/components/script/stylesheet_loader.rs
index 48e3016458e..9657227f91a 100644
--- a/components/script/stylesheet_loader.rs
+++ b/components/script/stylesheet_loader.rs
@@ -38,7 +38,7 @@ use std::sync::Mutex;
 use style::media_queries::MediaList;
 use style::parser::ParserContext;
 use style::shared_lock::{Locked, SharedRwLock};
-use style::stylesheets::import_rule::ImportSheet;
+use style::stylesheets::import_rule::{ImportLayer, ImportSheet};
 use style::stylesheets::StylesheetLoader as StyleStylesheetLoader;
 use style::stylesheets::{CssRules, ImportRule, Origin, Stylesheet, StylesheetContents};
 use style::values::CssUrl;
@@ -358,6 +358,7 @@ impl<'a> StyleStylesheetLoader for StylesheetLoader<'a> {
         context: &ParserContext,
         lock: &SharedRwLock,
         media: Arc<Locked<MediaList>>,
+        layer: Option<ImportLayer>,
     ) -> Arc<Locked<ImportRule>> {
         let sheet = Arc::new(Stylesheet {
             contents: StylesheetContents::from_shared_data(
@@ -374,8 +375,9 @@ impl<'a> StyleStylesheetLoader for StylesheetLoader<'a> {
         let stylesheet = ImportSheet(sheet.clone());
         let import = ImportRule {
             url,
-            source_location,
             stylesheet,
+            layer,
+            source_location,
         };
 
         let url = match import.url.url().cloned() {
diff --git a/components/style/applicable_declarations.rs b/components/style/applicable_declarations.rs
index 6353fb4c5e5..db0f7fce99f 100644
--- a/components/style/applicable_declarations.rs
+++ b/components/style/applicable_declarations.rs
@@ -6,6 +6,7 @@
 
 use crate::properties::PropertyDeclarationBlock;
 use crate::rule_tree::{CascadeLevel, StyleSource};
+use crate::stylesheets::layer_rule::LayerOrder;
 use crate::shared_lock::Locked;
 use servo_arc::Arc;
 use smallvec::SmallVec;
@@ -69,8 +70,10 @@ pub struct ApplicableDeclarationBlock {
     /// The bits containing the source order, cascade level, and shadow cascade
     /// order.
     bits: ApplicableDeclarationBits,
-    /// The specificity of the selector this block is represented by.
+    /// The specificity of the selector.
     pub specificity: u32,
+    /// The layer order of the selector.
+    pub layer_order: LayerOrder,
 }
 
 impl ApplicableDeclarationBlock {
@@ -85,16 +88,24 @@ impl ApplicableDeclarationBlock {
             source: StyleSource::from_declarations(declarations),
             bits: ApplicableDeclarationBits::new(0, level),
             specificity: 0,
+            layer_order: LayerOrder::root(),
         }
     }
 
-    /// Constructs an applicable declaration block from the given components
+    /// Constructs an applicable declaration block from the given components.
     #[inline]
-    pub fn new(source: StyleSource, order: u32, level: CascadeLevel, specificity: u32) -> Self {
+    pub fn new(
+        source: StyleSource,
+        source_order: u32,
+        level: CascadeLevel,
+        specificity: u32,
+        layer_order: LayerOrder,
+    ) -> Self {
         ApplicableDeclarationBlock {
             source,
-            bits: ApplicableDeclarationBits::new(order, level),
+            bits: ApplicableDeclarationBits::new(source_order, level),
             specificity,
+            layer_order,
         }
     }
 
diff --git a/components/style/bloom.rs b/components/style/bloom.rs
index 1840c780506..dc722b7bdba 100644
--- a/components/style/bloom.rs
+++ b/components/style/bloom.rs
@@ -8,6 +8,7 @@
 #![deny(missing_docs)]
 
 use crate::dom::{SendElement, TElement};
+use crate::LocalName;
 use atomic_refcell::{AtomicRefCell, AtomicRefMut};
 use owning_ref::OwningHandle;
 use selectors::bloom::BloomFilter;
@@ -107,9 +108,8 @@ impl<E: TElement> PushedElement<E> {
 /// We do this for attributes that are very common but not commonly used in
 /// selectors.
 #[inline]
-#[cfg(feature = "gecko")]
-pub fn is_attr_name_excluded_from_filter(atom: &crate::Atom) -> bool {
-    *atom == atom!("class") || *atom == atom!("id") || *atom == atom!("style")
+pub fn is_attr_name_excluded_from_filter(name: &LocalName) -> bool {
+    return *name == local_name!("class") || *name == local_name!("id") || *name == local_name!("style")
 }
 
 fn each_relevant_element_hash<E, F>(element: E, mut f: F)
@@ -126,14 +126,11 @@ where
 
     element.each_class(|class| f(class.get_hash()));
 
-    #[cfg(feature = "gecko")]
-    if static_prefs::pref!("layout.css.bloom-filter-attribute-names.enabled") {
-        element.each_attr_name(|name| {
-            if !is_attr_name_excluded_from_filter(name) {
-                f(name.get_hash())
-            }
-        });
-    }
+    element.each_attr_name(|name| {
+        if !is_attr_name_excluded_from_filter(name) {
+            f(name.get_hash())
+        }
+    });
 }
 
 impl<E: TElement> Drop for StyleBloom<E> {
diff --git a/components/style/custom_properties.rs b/components/style/custom_properties.rs
index df3f9660dd4..96328d9a51a 100644
--- a/components/style/custom_properties.rs
+++ b/components/style/custom_properties.rs
@@ -71,10 +71,29 @@ static ENVIRONMENT_VARIABLES: [EnvironmentVariable; 4] = [
     make_variable!(atom!("safe-area-inset-right"), get_safearea_inset_right),
 ];
 
+fn get_titlebar_radius(device: &Device) -> VariableValue {
+    VariableValue::pixel(device.titlebar_radius())
+}
+
+fn get_menu_radius(device: &Device) -> VariableValue {
+    VariableValue::pixel(device.menu_radius())
+}
+
+static CHROME_ENVIRONMENT_VARIABLES: [EnvironmentVariable; 2] = [
+    make_variable!(atom!("-moz-gtk-csd-titlebar-radius"), get_titlebar_radius),
+    make_variable!(atom!("-moz-gtk-menu-radius"), get_menu_radius),
+];
+
 impl CssEnvironment {
     #[inline]
     fn get(&self, name: &Atom, device: &Device) -> Option<VariableValue> {
-        let var = ENVIRONMENT_VARIABLES.iter().find(|var| var.name == *name)?;
+        if let Some(var) = ENVIRONMENT_VARIABLES.iter().find(|var| var.name == *name) {
+            return Some((var.evaluator)(device));
+        }
+        if !device.is_chrome_document() {
+            return None;
+        }
+        let var = CHROME_ENVIRONMENT_VARIABLES.iter().find(|var| var.name == *name)?;
         Some((var.evaluator)(device))
     }
 }
diff --git a/components/style/dom.rs b/components/style/dom.rs
index c85ff1edaff..308006efd11 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -520,10 +520,9 @@ pub trait TElement:
     }
 
     /// Internal iterator for the attribute names of this element.
-    #[cfg(feature = "gecko")]
     fn each_attr_name<F>(&self, callback: F)
     where
-        F: FnMut(&AtomIdent);
+        F: FnMut(&LocalName);
 
     /// Internal iterator for the part names that this element exports for a
     /// given part name.
diff --git a/components/style/gecko/arc_types.rs b/components/style/gecko/arc_types.rs
index 7aed10b6568..05fcdb5b12d 100644
--- a/components/style/gecko/arc_types.rs
+++ b/components/style/gecko/arc_types.rs
@@ -9,34 +9,24 @@
 #![allow(non_snake_case, missing_docs)]
 
 use crate::gecko::url::CssUrlData;
-use crate::gecko_bindings::structs::RawServoAnimationValue;
-use crate::gecko_bindings::structs::RawServoCounterStyleRule;
-use crate::gecko_bindings::structs::RawServoCssUrlData;
-use crate::gecko_bindings::structs::RawServoDeclarationBlock;
-use crate::gecko_bindings::structs::RawServoFontFaceRule;
-use crate::gecko_bindings::structs::RawServoFontFeatureValuesRule;
-use crate::gecko_bindings::structs::RawServoImportRule;
-use crate::gecko_bindings::structs::RawServoKeyframe;
-use crate::gecko_bindings::structs::RawServoKeyframesRule;
-use crate::gecko_bindings::structs::RawServoMediaList;
-use crate::gecko_bindings::structs::RawServoMediaRule;
-use crate::gecko_bindings::structs::RawServoMozDocumentRule;
-use crate::gecko_bindings::structs::RawServoNamespaceRule;
-use crate::gecko_bindings::structs::RawServoPageRule;
-use crate::gecko_bindings::structs::RawServoStyleRule;
-use crate::gecko_bindings::structs::RawServoStyleSheetContents;
-use crate::gecko_bindings::structs::RawServoSupportsRule;
-use crate::gecko_bindings::structs::ServoCssRules;
+use crate::gecko_bindings::structs::{
+    RawServoAnimationValue, RawServoCounterStyleRule, RawServoCssUrlData, RawServoDeclarationBlock,
+    RawServoFontFaceRule, RawServoFontFeatureValuesRule, RawServoImportRule, RawServoKeyframe,
+    RawServoKeyframesRule, RawServoLayerRule, RawServoMediaList, RawServoMediaRule,
+    RawServoMozDocumentRule, RawServoNamespaceRule, RawServoPageRule, RawServoScrollTimelineRule,
+    RawServoStyleRule, RawServoStyleSheetContents, RawServoSupportsRule, ServoCssRules,
+};
 use crate::gecko_bindings::sugar::ownership::{HasArcFFI, HasFFI, Strong};
 use crate::media_queries::MediaList;
 use crate::properties::animated_properties::AnimationValue;
 use crate::properties::{ComputedValues, PropertyDeclarationBlock};
 use crate::shared_lock::Locked;
 use crate::stylesheets::keyframes_rule::Keyframe;
-use crate::stylesheets::{CounterStyleRule, CssRules, FontFaceRule, FontFeatureValuesRule};
-use crate::stylesheets::{DocumentRule, ImportRule, KeyframesRule, MediaRule};
-use crate::stylesheets::{NamespaceRule, PageRule};
-use crate::stylesheets::{StyleRule, StylesheetContents, SupportsRule};
+use crate::stylesheets::{
+    CounterStyleRule, CssRules, DocumentRule, FontFaceRule, FontFeatureValuesRule, ImportRule,
+    KeyframesRule, LayerRule, MediaRule, NamespaceRule, PageRule, ScrollTimelineRule, StyleRule,
+    StylesheetContents, SupportsRule,
+};
 use servo_arc::{Arc, ArcBorrow};
 use std::{mem, ptr};
 
@@ -83,6 +73,9 @@ impl_arc_ffi!(Locked<Keyframe> => RawServoKeyframe
 impl_arc_ffi!(Locked<KeyframesRule> => RawServoKeyframesRule
               [Servo_KeyframesRule_AddRef, Servo_KeyframesRule_Release]);
 
+impl_arc_ffi!(Locked<LayerRule> => RawServoLayerRule
+              [Servo_LayerRule_AddRef, Servo_LayerRule_Release]);
+
 impl_arc_ffi!(Locked<MediaList> => RawServoMediaList
               [Servo_MediaList_AddRef, Servo_MediaList_Release]);
 
@@ -95,6 +88,9 @@ impl_arc_ffi!(Locked<NamespaceRule> => RawServoNamespaceRule
 impl_arc_ffi!(Locked<PageRule> => RawServoPageRule
               [Servo_PageRule_AddRef, Servo_PageRule_Release]);
 
+impl_arc_ffi!(Locked<ScrollTimelineRule> => RawServoScrollTimelineRule
+              [Servo_ScrollTimelineRule_AddRef, Servo_ScrollTimelineRule_Release]);
+
 impl_arc_ffi!(Locked<SupportsRule> => RawServoSupportsRule
               [Servo_SupportsRule_AddRef, Servo_SupportsRule_Release]);
 
diff --git a/components/style/gecko/media_features.rs b/components/style/gecko/media_features.rs
index 664a12f4108..40a34095d6f 100644
--- a/components/style/gecko/media_features.rs
+++ b/components/style/gecko/media_features.rs
@@ -406,45 +406,6 @@ fn eval_prefers_color_scheme(device: &Device, query_value: Option<PrefersColorSc
     }
 }
 
-/// Values for the -moz-toolbar-prefers-color-scheme media feature.
-#[derive(Clone, Copy, Debug, FromPrimitive, Parse, PartialEq, ToCss)]
-#[repr(u8)]
-enum ToolbarPrefersColorScheme {
-    Dark,
-    Light,
-    System,
-}
-
-/// The color-scheme of the toolbar in the current Firefox theme. This is based
-/// on a pref managed by the front-end.
-fn eval_toolbar_prefers_color_scheme(d: &Device, query_value: Option<ToolbarPrefersColorScheme>) -> bool {
-    let toolbar_value = match static_prefs::pref!("browser.theme.toolbar-theme") {
-        0 => ToolbarPrefersColorScheme::Dark,
-        1 => ToolbarPrefersColorScheme::Light,
-        _ => ToolbarPrefersColorScheme::System,
-    };
-
-    let query_value = match query_value {
-        Some(v) => v,
-        None => return true,
-    };
-
-    if query_value == toolbar_value {
-        return true;
-    }
-
-    if toolbar_value != ToolbarPrefersColorScheme::System {
-        return false;
-    }
-
-    // System might match light and dark as well.
-    match query_value {
-        ToolbarPrefersColorScheme::Dark => eval_prefers_color_scheme(d, Some(PrefersColorScheme::Dark)),
-        ToolbarPrefersColorScheme::Light => eval_prefers_color_scheme(d, Some(PrefersColorScheme::Light)),
-        ToolbarPrefersColorScheme::System => true,
-    }
-}
-
 bitflags! {
     /// https://drafts.csswg.org/mediaqueries-4/#mf-interaction
     struct PointerCapabilities: u8 {
@@ -607,6 +568,16 @@ fn eval_moz_windows_non_native_menus(
     query_value.map_or(use_non_native_menus, |v| v == use_non_native_menus)
 }
 
+fn eval_moz_overlay_scrollbars(
+    device: &Device,
+    query_value: Option<bool>,
+    _: Option<RangeOrOperator>,
+) -> bool {
+    let use_overlay =
+        unsafe { bindings::Gecko_MediaFeatures_UseOverlayScrollbars(device.document()) };
+    query_value.map_or(use_overlay, |v| v == use_overlay)
+}
+
 fn get_lnf_int(int_id: i32) -> i32 {
     unsafe { bindings::Gecko_GetLookAndFeelInt(int_id) }
 }
@@ -680,7 +651,7 @@ macro_rules! bool_pref_feature {
 /// to support new types in these entries and (2) ensuring that either
 /// nsPresContext::MediaFeatureValuesChanged is called when the value that
 /// would be returned by the evaluator function could change.
-pub static MEDIA_FEATURES: [MediaFeatureDescription; 60] = [
+pub static MEDIA_FEATURES: [MediaFeatureDescription; 58] = [
     feature!(
         atom!("width"),
         AllowsRanges::Yes,
@@ -890,37 +861,35 @@ pub static MEDIA_FEATURES: [MediaFeatureDescription; 60] = [
         Evaluator::BoolInteger(eval_moz_non_native_content_theme),
         ParsingRequirements::CHROME_AND_UA_ONLY,
     ),
-    feature!(
-        atom!("-moz-toolbar-prefers-color-scheme"),
-        AllowsRanges::No,
-        keyword_evaluator!(eval_toolbar_prefers_color_scheme, ToolbarPrefersColorScheme),
-        ParsingRequirements::CHROME_AND_UA_ONLY,
-    ),
     feature!(
         atom!("-moz-windows-non-native-menus"),
         AllowsRanges::No,
         Evaluator::BoolInteger(eval_moz_windows_non_native_menus),
         ParsingRequirements::CHROME_AND_UA_ONLY,
     ),
+    feature!(
+        atom!("-moz-overlay-scrollbars"),
+        AllowsRanges::No,
+        Evaluator::BoolInteger(eval_moz_overlay_scrollbars),
+        ParsingRequirements::CHROME_AND_UA_ONLY,
+    ),
 
     lnf_int_feature!(atom!("-moz-scrollbar-start-backward"), ScrollArrowStyle, get_scrollbar_start_backward),
     lnf_int_feature!(atom!("-moz-scrollbar-start-forward"), ScrollArrowStyle, get_scrollbar_start_forward),
     lnf_int_feature!(atom!("-moz-scrollbar-end-backward"), ScrollArrowStyle, get_scrollbar_end_backward),
     lnf_int_feature!(atom!("-moz-scrollbar-end-forward"), ScrollArrowStyle, get_scrollbar_end_forward),
     lnf_int_feature!(atom!("-moz-scrollbar-thumb-proportional"), ScrollSliderStyle),
-    lnf_int_feature!(atom!("-moz-overlay-scrollbars"), UseOverlayScrollbars),
     lnf_int_feature!(atom!("-moz-menubar-drag"), MenuBarDrag),
     lnf_int_feature!(atom!("-moz-windows-default-theme"), WindowsDefaultTheme),
     lnf_int_feature!(atom!("-moz-mac-graphite-theme"), MacGraphiteTheme),
     lnf_int_feature!(atom!("-moz-mac-big-sur-theme"), MacBigSurTheme),
+    lnf_int_feature!(atom!("-moz-mac-rtl"), MacRTL),
     lnf_int_feature!(atom!("-moz-windows-accent-color-in-titlebar"), WindowsAccentColorInTitlebar),
     lnf_int_feature!(atom!("-moz-windows-compositor"), DWMCompositor),
     lnf_int_feature!(atom!("-moz-windows-classic"), WindowsClassic),
     lnf_int_feature!(atom!("-moz-windows-glass"), WindowsGlass),
     lnf_int_feature!(atom!("-moz-swipe-animation-enabled"), SwipeAnimationEnabled),
     lnf_int_feature!(atom!("-moz-gtk-csd-available"), GTKCSDAvailable),
-    lnf_int_feature!(atom!("-moz-gtk-csd-hide-titlebar-by-default"), GTKCSDHideTitlebarByDefault),
-    lnf_int_feature!(atom!("-moz-gtk-csd-transparent-background"), GTKCSDTransparentBackground),
     lnf_int_feature!(atom!("-moz-gtk-csd-minimize-button"), GTKCSDMinimizeButton),
     lnf_int_feature!(atom!("-moz-gtk-csd-maximize-button"), GTKCSDMaximizeButton),
     lnf_int_feature!(atom!("-moz-gtk-csd-close-button"), GTKCSDCloseButton),
diff --git a/components/style/gecko/media_queries.rs b/components/style/gecko/media_queries.rs
index 2f5a9673d4c..7bab0b002d1 100644
--- a/components/style/gecko/media_queries.rs
+++ b/components/style/gecko/media_queries.rs
@@ -93,7 +93,9 @@ impl Device {
             document,
             default_values: ComputedValues::default_values(doc),
             root_font_size: AtomicU32::new(FONT_MEDIUM_PX.to_bits()),
-            body_text_color: AtomicUsize::new(prefs.mColors.mDefault as usize),
+            // This gets updated when we see the <body>, so it doesn't really
+            // matter which color-scheme we look at here.
+            body_text_color: AtomicUsize::new(prefs.mLightColors.mDefault as usize),
             used_root_font_size: AtomicBool::new(false),
             used_font_metrics: AtomicBool::new(false),
             used_viewport_size: AtomicBool::new(false),
@@ -386,13 +388,18 @@ impl Device {
     }
 
     /// Returns the default background color.
-    pub fn default_background_color(&self) -> RGBA {
-        convert_nscolor_to_rgba(self.pref_sheet_prefs().mColors.mDefaultBackground)
+    ///
+    /// This is only for forced-colors/high-contrast, so looking at light colors
+    /// is ok.
+    pub fn default_background_color_for_forced_colors(&self) -> RGBA {
+        convert_nscolor_to_rgba(self.pref_sheet_prefs().mLightColors.mDefaultBackground)
     }
 
     /// Returns the default foreground color.
-    pub fn default_color(&self) -> RGBA {
-        convert_nscolor_to_rgba(self.pref_sheet_prefs().mColors.mDefault)
+    ///
+    /// See above for looking at light colors only.
+    pub fn default_color_for_forced_colors(&self) -> RGBA {
+        convert_nscolor_to_rgba(self.pref_sheet_prefs().mLightColors.mDefault)
     }
 
     /// Returns the current effective text zoom.
@@ -439,4 +446,24 @@ impl Device {
             bindings::Gecko_IsSupportedImageMimeType(mime_type.as_ptr(), mime_type.len() as u32)
         }
     }
+
+    /// Returns the gtk titlebar radius in CSS pixels.
+    pub fn titlebar_radius(&self) -> f32 {
+        unsafe {
+            bindings::Gecko_GetLookAndFeelInt(bindings::LookAndFeel_IntID::TitlebarRadius as i32) as f32
+        }
+    }
+
+    /// Returns the gtk menu radius in CSS pixels.
+    pub fn menu_radius(&self) -> f32 {
+        unsafe {
+            bindings::Gecko_GetLookAndFeelInt(bindings::LookAndFeel_IntID::GtkMenuRadius as i32) as f32
+        }
+    }
+
+    /// Return whether the document is a chrome document.
+    #[inline]
+    pub fn is_chrome_document(&self) -> bool {
+        self.pref_sheet_prefs().mIsChrome
+    }
 }
diff --git a/components/style/gecko/selector_parser.rs b/components/style/gecko/selector_parser.rs
index 5379454daa0..7028d554aa2 100644
--- a/components/style/gecko/selector_parser.rs
+++ b/components/style/gecko/selector_parser.rs
@@ -249,8 +249,7 @@ impl ::selectors::SelectorImpl for SelectorImpl {
     type NonTSPseudoClass = NonTSPseudoClass;
 
     fn should_collect_attr_hash(name: &AtomIdent) -> bool {
-        static_prefs::pref!("layout.css.bloom-filter-attribute-names.enabled") &&
-            !crate::bloom::is_attr_name_excluded_from_filter(name)
+        !crate::bloom::is_attr_name_excluded_from_filter(name)
     }
 }
 
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 476e710795d..95da8b986f6 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -1016,7 +1016,7 @@ impl<'le> TElement for GeckoElement<'le> {
     #[inline]
     fn namespace(&self) -> &WeakNamespace {
         unsafe {
-            let namespace_manager = structs::nsContentUtils_sNameSpaceManager;
+            let namespace_manager = structs::nsNameSpaceManager_sInstance.mRawPtr;
             WeakNamespace::new((*namespace_manager).mURIArray[self.namespace_id() as usize].mRawPtr)
         }
     }
diff --git a/components/style/invalidation/stylesheets.rs b/components/style/invalidation/stylesheets.rs
index edecc3fc243..fc39b8cca76 100644
--- a/components/style/invalidation/stylesheets.rs
+++ b/components/style/invalidation/stylesheets.rs
@@ -543,6 +543,7 @@ impl StylesheetInvalidationSet {
             FontFeatureValues(..) |
             FontFace(..) |
             Keyframes(..) |
+            ScrollTimeline(..) |
             Style(..) => {
                 if is_generic_change {
                     // TODO(emilio): We need to do this for selector / keyframe
@@ -555,7 +556,7 @@ impl StylesheetInvalidationSet {
 
                 self.collect_invalidations_for_rule(rule, guard, device, quirks_mode)
             },
-            Document(..) | Import(..) | Media(..) | Supports(..) => {
+            Document(..) | Import(..) | Media(..) | Supports(..) | Layer(..) => {
                 if !is_generic_change &&
                     !EffectiveRules::is_effective(guard, device, quirks_mode, rule)
                 {
@@ -596,7 +597,7 @@ impl StylesheetInvalidationSet {
                     }
                 }
             },
-            Document(..) | Namespace(..) | Import(..) | Media(..) | Supports(..) => {
+            Document(..) | Namespace(..) | Import(..) | Media(..) | Supports(..) | Layer(..) => {
                 // Do nothing, relevant nested rules are visited as part of the
                 // iteration.
             },
@@ -618,6 +619,10 @@ impl StylesheetInvalidationSet {
                     // existing elements.
                 }
             },
+            ScrollTimeline(..) => {
+                // TODO: Bug 1676784: check if animation-timeline name is referenced.
+                // Now we do nothing.
+            },
             CounterStyle(..) | Page(..) | Viewport(..) | FontFeatureValues(..) => {
                 debug!(
                     " > Found unsupported rule, marking the whole subtree \
diff --git a/components/style/properties/cascade.rs b/components/style/properties/cascade.rs
index e3f919cc56e..fe5bae33fe8 100644
--- a/components/style/properties/cascade.rs
+++ b/components/style/properties/cascade.rs
@@ -430,7 +430,7 @@ fn tweak_when_ignoring_colors(
             // widget background color's rgb channels but not alpha...
             let alpha = alpha_channel(color, context);
             if alpha != 0 {
-                let mut color = context.builder.device.default_background_color();
+                let mut color = context.builder.device.default_background_color_for_forced_colors();
                 color.alpha = alpha;
                 declarations_to_apply_unless_overriden
                     .push(PropertyDeclaration::BackgroundColor(color.into()))
@@ -448,7 +448,7 @@ fn tweak_when_ignoring_colors(
             // override this with a non-transparent color, then override it with
             // the default color. Otherwise just let it inherit through.
             if context.builder.get_parent_inherited_text().clone_color().alpha == 0 {
-                let color = context.builder.device.default_color();
+                let color = context.builder.device.default_color_for_forced_colors();
                 declarations_to_apply_unless_overriden.push(PropertyDeclaration::Color(
                     specified::ColorPropertyValue(color.into()),
                 ))
@@ -794,12 +794,6 @@ impl<'a, 'b: 'a> Cascade<'a, 'b> {
         {
             builder.add_flags(ComputedValueFlags::HAS_AUTHOR_SPECIFIED_BORDER_BACKGROUND);
         }
-        if self
-            .author_specified
-            .contains_any(LonghandIdSet::padding_properties())
-        {
-            builder.add_flags(ComputedValueFlags::HAS_AUTHOR_SPECIFIED_PADDING);
-        }
 
         if self
             .author_specified
@@ -859,13 +853,20 @@ impl<'a, 'b: 'a> Cascade<'a, 'b> {
 
         // We're using the same reset style as another element, and we'll skip
         // applying the relevant properties. So we need to do the relevant
-        // bookkeeping here to keep these two bits correct.
+        // bookkeeping here to keep these bits correct.
         //
-        // Note that all the properties involved are non-inherited, so we don't
-        // need to do anything else other than just copying the bits over.
-        let reset_props_bits = ComputedValueFlags::HAS_AUTHOR_SPECIFIED_BORDER_BACKGROUND |
-            ComputedValueFlags::HAS_AUTHOR_SPECIFIED_PADDING;
-        builder.add_flags(cached_style.flags & reset_props_bits);
+        // Note that the border/background properties are non-inherited, so we
+        // don't need to do anything else other than just copying the bits over.
+        //
+        // When using this optimization, we also need to copy whether the old
+        // style specified viewport units / used font-relative lengths, this one
+        // would as well.  It matches the same rules, so it is the right thing
+        // to do anyways, even if it's only used on inherited properties.
+        let bits_to_copy = ComputedValueFlags::HAS_AUTHOR_SPECIFIED_BORDER_BACKGROUND |
+            ComputedValueFlags::DEPENDS_ON_SELF_FONT_METRICS |
+            ComputedValueFlags::DEPENDS_ON_INHERITED_FONT_METRICS |
+            ComputedValueFlags::USES_VIEWPORT_UNITS;
+        builder.add_flags(cached_style.flags & bits_to_copy);
 
         true
     }
@@ -916,12 +917,7 @@ impl<'a, 'b: 'a> Cascade<'a, 'b> {
             // we have a generic family to actually replace it with.
             let prioritize_user_fonts = !use_document_fonts &&
                 default_font_type != GenericFontFamily::None &&
-                matches!(
-                    generic,
-                    GenericFontFamily::None |
-                    GenericFontFamily::Fantasy |
-                    GenericFontFamily::Cursive
-                );
+                !generic.valid_for_user_font_prioritization();
 
             if !prioritize_user_fonts && default_font_type == font.mFont.family.families.fallback {
                 // Nothing to do.
diff --git a/components/style/properties/computed_value_flags.rs b/components/style/properties/computed_value_flags.rs
index 0942bbd0d13..25adc3327ca 100644
--- a/components/style/properties/computed_value_flags.rs
+++ b/components/style/properties/computed_value_flags.rs
@@ -84,12 +84,6 @@ bitflags! {
         /// https://github.com/w3c/csswg-drafts/issues/4777#issuecomment-604424845
         const HAS_AUTHOR_SPECIFIED_BORDER_BACKGROUND = 1 << 14;
 
-        /// Whether there are author-specified rules for padding-* properties.
-        ///
-        /// FIXME(emilio): Try to merge this with BORDER_BACKGROUND, see
-        /// https://github.com/w3c/csswg-drafts/issues/4777
-        const HAS_AUTHOR_SPECIFIED_PADDING = 1 << 15;
-
         /// Whether there are author-specified rules for `font-family`.
         const HAS_AUTHOR_SPECIFIED_FONT_FAMILY = 1 << 16;
 
diff --git a/components/style/properties/data.py b/components/style/properties/data.py
index b2ab520ad3d..49157860fbb 100644
--- a/components/style/properties/data.py
+++ b/components/style/properties/data.py
@@ -476,6 +476,7 @@ class Longhand(Property):
                 "FontWeight",
                 "GreaterThanOrEqualToOneNumber",
                 "GridAutoFlow",
+                "ImageRendering",
                 "InitialLetter",
                 "Integer",
                 "JustifyContent",
diff --git a/components/style/properties/gecko.mako.rs b/components/style/properties/gecko.mako.rs
index 86cdeefa982..bc0a78183a8 100644
--- a/components/style/properties/gecko.mako.rs
+++ b/components/style/properties/gecko.mako.rs
@@ -1049,7 +1049,7 @@ fn static_assert() {
             % if member:
             ours.m${gecko_ffi_name}.${member} = others.m${gecko_ffi_name}.${member};
             % else:
-            ours.m${gecko_ffi_name} = others.m${gecko_ffi_name};
+            ours.m${gecko_ffi_name} = others.m${gecko_ffi_name}.clone();
             % endif
         }
     }
@@ -1183,7 +1183,7 @@ fn static_assert() {
 <% skip_box_longhands= """display
                           animation-name animation-delay animation-duration
                           animation-direction animation-fill-mode animation-play-state
-                          animation-iteration-count animation-timing-function
+                          animation-iteration-count animation-timeline animation-timing-function
                           clear transition-duration transition-delay
                           transition-timing-function transition-property
                           -webkit-line-clamp""" %>
@@ -1445,6 +1445,27 @@ fn static_assert() {
     ${impl_copy_animation_value('iteration_count', 'IterationCount')}
     ${impl_animation_or_transition_timing_function('animation')}
 
+    pub fn set_animation_timeline<I>(&mut self, v: I)
+    where
+        I: IntoIterator<Item = longhands::animation_timeline::computed_value::single_value::T>,
+        I::IntoIter: ExactSizeIterator
+    {
+        let v = v.into_iter();
+        debug_assert_ne!(v.len(), 0);
+        let input_len = v.len();
+        self.gecko.mAnimations.ensure_len(input_len);
+
+        self.gecko.mAnimationTimelineCount = input_len as u32;
+        for (gecko, servo) in self.gecko.mAnimations.iter_mut().take(input_len as usize).zip(v) {
+            gecko.mTimeline = servo;
+        }
+    }
+    pub fn animation_timeline_at(&self, index: usize) -> values::specified::box_::AnimationTimeline {
+        self.gecko.mAnimations[index].mTimeline.clone()
+    }
+    ${impl_animation_count('timeline', 'Timeline')}
+    ${impl_copy_animation_value('timeline', 'Timeline')}
+
     #[allow(non_snake_case)]
     pub fn set__webkit_line_clamp(&mut self, v: longhands::_webkit_line_clamp::computed_value::T) {
         self.gecko.mLineClamp = match v {
diff --git a/components/style/properties/longhands/box.mako.rs b/components/style/properties/longhands/box.mako.rs
index c8942fdc6ff..62c3da725f5 100644
--- a/components/style/properties/longhands/box.mako.rs
+++ b/components/style/properties/longhands/box.mako.rs
@@ -320,6 +320,22 @@ ${helpers.predefined_type(
     rule_types_allowed=DEFAULT_RULES_EXCEPT_KEYFRAME,
 )}
 
+${helpers.predefined_type(
+    "animation-timeline",
+    "AnimationTimeline",
+    "computed::AnimationTimeline::auto()",
+    engines="gecko servo-2013 servo-2020",
+    servo_2013_pref="layout.2013.unimplemented",
+    servo_2020_pref="layout.2020.unimplemented",
+    initial_specified_value="specified::AnimationTimeline::auto()",
+    vector=True,
+    need_index=True,
+    animation_value_type="none",
+    gecko_pref="layout.css.scroll-linked-animations.enabled",
+    spec="https://drafts.csswg.org/css-animations-2/#propdef-animation-timeline",
+    rule_types_allowed=DEFAULT_RULES_EXCEPT_KEYFRAME,
+)}
+
 <% transform_extra_prefixes = "moz:layout.css.prefixes.transforms webkit" %>
 
 ${helpers.predefined_type(
diff --git a/components/style/properties/longhands/inherited_box.mako.rs b/components/style/properties/longhands/inherited_box.mako.rs
index cea3fde021a..294f36bd96f 100644
--- a/components/style/properties/longhands/inherited_box.mako.rs
+++ b/components/style/properties/longhands/inherited_box.mako.rs
@@ -69,17 +69,13 @@ ${helpers.single_keyword(
 
 // According to to CSS-IMAGES-3, `optimizespeed` and `optimizequality` are synonyms for `auto`
 // And, firefox doesn't support `pixelated` yet (https://bugzilla.mozilla.org/show_bug.cgi?id=856337)
-${helpers.single_keyword(
+${helpers.predefined_type(
     "image-rendering",
-    "auto crisp-edges",
+    "ImageRendering",
+    "computed::ImageRendering::Auto",
     engines="gecko servo-2013 servo-2020",
-    extra_gecko_values="optimizespeed optimizequality",
-    extra_servo_2013_values="pixelated",
-    extra_servo_2020_values="pixelated",
-    gecko_aliases="-moz-crisp-edges=crisp-edges",
-    gecko_enum_prefix="StyleImageRendering",
-    animation_value_type="discrete",
     spec="https://drafts.csswg.org/css-images/#propdef-image-rendering",
+    animation_value_type="discrete",
 )}
 
 ${helpers.single_keyword(
diff --git a/components/style/properties/longhands/inherited_ui.mako.rs b/components/style/properties/longhands/inherited_ui.mako.rs
index badcc3f2012..44dfdd74a2a 100644
--- a/components/style/properties/longhands/inherited_ui.mako.rs
+++ b/components/style/properties/longhands/inherited_ui.mako.rs
@@ -104,6 +104,8 @@ ${helpers.predefined_type(
     gecko_pref="layout.css.color-scheme.enabled",
     animation_value_type="discrete",
     has_effect_on_gecko_scrollbars=False,
+    ignored_when_colors_disabled=True,
+    enabled_in="chrome",
 )}
 
 ${helpers.predefined_type(
diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index 67183062e2f..4ee388dfe6c 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -945,18 +945,6 @@ impl LonghandIdSet {
         &HAS_NO_EFFECT_ON_SCROLLBARS
     }
 
-    /// Returns the set of padding properties for the purpose of disabling
-    /// native appearance.
-    #[inline]
-    pub fn padding_properties() -> &'static Self {
-        <% assert "padding" in logical_groups %>
-        ${static_longhand_id_set(
-            "PADDING_PROPERTIES",
-            lambda p: p.logical_group == "padding"
-        )}
-        &PADDING_PROPERTIES
-    }
-
     /// Returns the set of border properties for the purpose of disabling native
     /// appearance.
     #[inline]
@@ -2921,7 +2909,7 @@ pub mod style_structs {
             }
 
             /// Returns true if animation properties are equal between styles, but without
-            /// considering keyframe data.
+            /// considering keyframe data and animation-timeline.
             #[cfg(feature = "servo")]
             pub fn animations_equals(&self, other: &Self) -> bool {
                 self.animation_name_iter().eq(other.animation_name_iter()) &&
@@ -3098,27 +3086,76 @@ impl ComputedValues {
     ///
     /// Note that the value will usually be the computed value, except for
     /// colors, where it's resolved.
-    pub fn get_longhand_property_value<W>(
+    ///
+    /// TODO(emilio): We should move all the special resolution from
+    /// nsComputedDOMStyle to ToResolvedValue instead.
+    pub fn get_resolved_value(
         &self,
         property_id: LonghandId,
-        dest: &mut CssWriter<W>
-    ) -> fmt::Result
-    where
-        W: Write,
-    {
+        dest: &mut CssStringWriter,
+    ) -> fmt::Result {
         use crate::values::resolved::ToResolvedValue;
 
+        let mut dest = CssWriter::new(dest);
+        let context = resolved::Context {
+            style: self,
+        };
+        match property_id {
+            % for specified_type, props in groupby(data.longhands, key=lambda x: x.specified_type()):
+            <% props = list(props) %>
+            ${" |\n".join("LonghandId::{}".format(p.camel_case) for p in props)} => {
+                let value = match property_id {
+                    % for prop in props:
+                    LonghandId::${prop.camel_case} => self.clone_${prop.ident}(),
+                    % endfor
+                    _ => unsafe { debug_unreachable!() },
+                };
+                value.to_resolved_value(&context).to_css(&mut dest)
+            }
+            % endfor
+        }
+    }
+
+    /// Returns the given longhand's resolved value as a property declaration.
+    pub fn resolved_declaration(&self, property_id: LonghandId) -> PropertyDeclaration {
+        use crate::values::resolved::ToResolvedValue;
+        use crate::values::computed::ToComputedValue;
+
         let context = resolved::Context {
             style: self,
         };
 
-        // TODO(emilio): Is it worth to merge branches here just like
-        // PropertyDeclaration::to_css does?
         match property_id {
-            % for prop in data.longhands:
-            LonghandId::${prop.camel_case} => {
-                let value = self.clone_${prop.ident}();
-                value.to_resolved_value(&context).to_css(dest)
+            % for specified_type, props in groupby(data.longhands, key=lambda x: x.specified_type()):
+            <% props = list(props) %>
+            ${" |\n".join("LonghandId::{}".format(p.camel_case) for p in props)} => {
+                let value = match property_id {
+                    % for prop in props:
+                    LonghandId::${prop.camel_case} => self.clone_${prop.ident}(),
+                    % endfor
+                    _ => unsafe { debug_unreachable!() },
+                };
+                let resolved = value.to_resolved_value(&context);
+                let computed = ToResolvedValue::from_resolved_value(resolved);
+                let specified = ToComputedValue::from_computed_value(&computed);
+                % if props[0].boxed:
+                let specified = Box::new(specified);
+                % endif
+                % if len(props) == 1:
+                PropertyDeclaration::${props[0].camel_case}(specified)
+                % else:
+                unsafe {
+                    let mut out = mem::MaybeUninit::uninit();
+                    ptr::write(
+                        out.as_mut_ptr() as *mut PropertyDeclarationVariantRepr<${specified_type}>,
+                        PropertyDeclarationVariantRepr {
+                            tag: property_id as u16,
+                            value: specified,
+                        },
+                    );
+                    out.assume_init()
+                }
+                % endif
             }
             % endfor
         }
@@ -3195,9 +3232,9 @@ impl ComputedValues {
         match property {
             PropertyDeclarationId::Longhand(id) => {
                 let mut s = String::new();
-                self.get_longhand_property_value(
+                self.get_resolved_value(
                     id,
-                    &mut CssWriter::new(&mut s)
+                    &mut s
                 ).unwrap();
                 s
             }
diff --git a/components/style/properties/shorthands/box.mako.rs b/components/style/properties/shorthands/box.mako.rs
index 4beef9a0d95..5cd52902ec9 100644
--- a/components/style/properties/shorthands/box.mako.rs
+++ b/components/style/properties/shorthands/box.mako.rs
@@ -189,11 +189,11 @@ macro_rules! try_parse_one {
                     sub_properties="animation-name animation-duration
                                     animation-timing-function animation-delay
                                     animation-iteration-count animation-direction
-                                    animation-fill-mode animation-play-state"
+                                    animation-fill-mode animation-play-state animation-timeline"
                     rule_types_allowed="Style"
                     spec="https://drafts.csswg.org/css-animations/#propdef-animation">
     <%
-        props = "name duration timing_function delay iteration_count \
+        props = "name timeline duration timing_function delay iteration_count \
                  direction fill_mode play_state".split()
     %>
     % for prop in props:
@@ -210,6 +210,13 @@ macro_rules! try_parse_one {
             % endfor
         }
 
+        fn scroll_linked_animations_enabled() -> bool {
+            #[cfg(feature = "gecko")]
+            return static_prefs::pref!("layout.css.scroll-linked-animations.enabled");
+            #[cfg(feature = "servo")]
+            return false;
+        }
+
         fn parse_one_animation<'i, 't>(
             context: &ParserContext,
             input: &mut Parser<'i, 't>,
@@ -234,6 +241,9 @@ macro_rules! try_parse_one {
                 try_parse_one!(context, input, fill_mode, animation_fill_mode);
                 try_parse_one!(context, input, play_state, animation_play_state);
                 try_parse_one!(context, input, name, animation_name);
+                if scroll_linked_animations_enabled() {
+                    try_parse_one!(context, input, timeline, animation_timeline);
+                }
 
                 parsed -= 1;
                 break
@@ -280,22 +290,46 @@ macro_rules! try_parse_one {
 
             // If any value list length is differs then we don't do a shorthand serialization
             // either.
-            % for name in props[1:]:
+            % for name in props[2:]:
                 if len != self.animation_${name}.0.len() {
                     return Ok(())
                 }
             % endfor
 
+            // If the preference of animation-timeline is disabled, `self.animation_timeline` is
+            // None.
+            if self.animation_timeline.map_or(false, |v| len != v.0.len()) {
+                return Ok(());
+            }
+
             for i in 0..len {
                 if i != 0 {
                     dest.write_str(", ")?;
                 }
 
-                % for name in props[1:]:
+                % for name in props[2:]:
                     self.animation_${name}.0[i].to_css(dest)?;
                     dest.write_str(" ")?;
                 % endfor
+
                 self.animation_name.0[i].to_css(dest)?;
+
+                // Based on the spec, the default values of other properties must be output in at
+                // least the cases necessary to distinguish an animation-name. The serialization
+                // order of animation-timeline is always later than animation-name, so it's fine
+                // to not serialize it if it is the default value. It's still possible to
+                // distinguish them (because we always serialize animation-name).
+                // https://drafts.csswg.org/css-animations-1/#animation
+                // https://drafts.csswg.org/css-animations-2/#typedef-single-animation
+                //
+                // Note: it's also fine to always serialize this. However, it seems Blink
+                // doesn't serialize default animation-timeline now, so we follow the same rule.
+                if let Some(ref timeline) = self.animation_timeline {
+                    if !timeline.0[i].is_auto() {
+                        dest.write_char(' ')?;
+                        timeline.0[i].to_css(dest)?;
+                    }
+                }
             }
             Ok(())
         }
diff --git a/components/style/properties/shorthands/list.mako.rs b/components/style/properties/shorthands/list.mako.rs
index f757c368cfa..0ba42d3fca4 100644
--- a/components/style/properties/shorthands/list.mako.rs
+++ b/components/style/properties/shorthands/list.mako.rs
@@ -7,7 +7,6 @@
 <%helpers:shorthand name="list-style"
                     engines="gecko servo-2013 servo-2020"
                     sub_properties="list-style-position list-style-image list-style-type"
-                    derive_serialize="True"
                     spec="https://drafts.csswg.org/css-lists/#propdef-list-style">
     use crate::properties::longhands::{list_style_image, list_style_position, list_style_type};
     use crate::values::specified::Image;
@@ -104,4 +103,43 @@
             _ => Err(input.new_custom_error(StyleParseErrorKind::UnspecifiedError)),
         }
     }
+
+    impl<'a> ToCss for LonghandsToSerialize<'a>  {
+        fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
+            use longhands::list_style_position::SpecifiedValue as ListStylePosition;
+            use longhands::list_style_type::SpecifiedValue as ListStyleType;
+            use longhands::list_style_image::SpecifiedValue as ListStyleImage;
+            let mut have_one_non_initial_value = false;
+            #[cfg(any(feature = "gecko", feature = "servo-layout-2013"))]
+            let position_is_initial = self.list_style_position == &ListStylePosition::Outside;
+            #[cfg(feature = "servo-layout-2020")]
+            let position_is_initial = self.list_style_position == Some(&ListStylePosition::Outside);
+            if !position_is_initial {
+                self.list_style_position.to_css(dest)?;
+                have_one_non_initial_value = true;
+            }
+            if self.list_style_image != &ListStyleImage::None {
+                if have_one_non_initial_value {
+                    dest.write_str(" ")?;
+                }
+                self.list_style_image.to_css(dest)?;
+                have_one_non_initial_value = true;
+            }
+            #[cfg(feature = "gecko")]
+            let type_is_initial = self.list_style_type == &ListStyleType::disc();
+            #[cfg(feature = "servo")]
+            let type_is_initial = self.list_style_type == &ListStyleType::Disc;
+            if !type_is_initial {
+                if have_one_non_initial_value {
+                    dest.write_str(" ")?;
+                }
+                self.list_style_type.to_css(dest)?;
+                have_one_non_initial_value = true;
+            }
+            if !have_one_non_initial_value {
+                self.list_style_position.to_css(dest)?;
+            }
+            Ok(())
+        }
+    }
 </%helpers:shorthand>
diff --git a/components/style/properties/shorthands/text.mako.rs b/components/style/properties/shorthands/text.mako.rs
index 03637d557e5..eb0c5dc4003 100644
--- a/components/style/properties/shorthands/text.mako.rs
+++ b/components/style/properties/shorthands/text.mako.rs
@@ -89,6 +89,14 @@
             }
 
             % if engine == "gecko":
+            if !is_auto_thickness {
+                if has_value {
+                    dest.write_str(" ")?;
+                }
+                self.text_decoration_thickness.to_css(dest)?;
+                has_value = true;
+            }
+
             if !is_solid_style {
                 if has_value {
                     dest.write_str(" ")?;
@@ -104,13 +112,6 @@
                 self.text_decoration_color.to_css(dest)?;
                 has_value = true;
             }
-
-            if !is_auto_thickness {
-                if has_value {
-                    dest.write_str(" ")?;
-                }
-                self.text_decoration_thickness.to_css(dest)?;
-            }
             % endif
 
             Ok(())
diff --git a/components/style/rule_collector.rs b/components/style/rule_collector.rs
index 86293e5c785..65f55ea887a 100644
--- a/components/style/rule_collector.rs
+++ b/components/style/rule_collector.rs
@@ -12,7 +12,7 @@ use crate::selector_map::SelectorMap;
 use crate::selector_parser::PseudoElement;
 use crate::shared_lock::Locked;
 use crate::stylesheets::Origin;
-use crate::stylist::{AuthorStylesEnabled, Rule, RuleInclusion, Stylist};
+use crate::stylist::{AuthorStylesEnabled, CascadeData, Rule, RuleInclusion, Stylist};
 use selectors::matching::{ElementSelectorFlags, MatchingContext, MatchingMode};
 use servo_arc::ArcBorrow;
 use smallvec::SmallVec;
@@ -148,7 +148,7 @@ where
         f(self);
         if start != self.rules.len() {
             self.rules[start..]
-                .sort_unstable_by_key(|block| (block.specificity, block.source_order()));
+                .sort_unstable_by_key(|block| (block.layer_order, block.specificity, block.source_order()));
         }
         self.context.current_host = old_host;
         self.in_sort_scope = false;
@@ -173,7 +173,7 @@ where
         };
 
         self.in_tree(None, |collector| {
-            collector.collect_rules_in_map(map, cascade_level);
+            collector.collect_rules_in_map(map, cascade_level, cascade_data);
         });
     }
 
@@ -214,7 +214,7 @@ where
     }
 
     #[inline]
-    fn collect_rules_in_list(&mut self, part_rules: &[Rule], cascade_level: CascadeLevel) {
+    fn collect_rules_in_list(&mut self, part_rules: &[Rule], cascade_level: CascadeLevel, cascade_data: &CascadeData) {
         debug_assert!(self.in_sort_scope, "Rules gotta be sorted");
         SelectorMap::get_matching_rules(
             self.element,
@@ -223,11 +223,12 @@ where
             &mut self.context,
             &mut self.flags_setter,
             cascade_level,
+            cascade_data,
         );
     }
 
     #[inline]
-    fn collect_rules_in_map(&mut self, map: &SelectorMap<Rule>, cascade_level: CascadeLevel) {
+    fn collect_rules_in_map(&mut self, map: &SelectorMap<Rule>, cascade_level: CascadeLevel, cascade_data: &CascadeData) {
         debug_assert!(self.in_sort_scope, "Rules gotta be sorted");
         map.get_all_matching_rules(
             self.element,
@@ -236,6 +237,7 @@ where
             &mut self.context,
             &mut self.flags_setter,
             cascade_level,
+            cascade_data,
         );
     }
 
@@ -277,7 +279,7 @@ where
                 let cascade_level = CascadeLevel::AuthorNormal {
                     shadow_cascade_order,
                 };
-                collector.collect_rules_in_map(slotted_rules, cascade_level);
+                collector.collect_rules_in_map(slotted_rules, cascade_level, data);
             });
         }
     }
@@ -303,7 +305,7 @@ where
         let cascade_level = CascadeLevel::same_tree_author_normal();
         self.in_shadow_tree(containing_shadow.host(), |collector| {
             if let Some(map) = cascade_data.normal_rules(collector.pseudo_element) {
-                collector.collect_rules_in_map(map, cascade_level);
+                collector.collect_rules_in_map(map, cascade_level, cascade_data);
             }
 
             // Collect rules from :host::part() and such
@@ -319,7 +321,7 @@ where
 
             hash_target.each_part(|part| {
                 if let Some(part_rules) = part_rules.get(&part.0) {
-                    collector.collect_rules_in_list(part_rules, cascade_level);
+                    collector.collect_rules_in_list(part_rules, cascade_level, cascade_data);
                 }
             });
         });
@@ -352,7 +354,7 @@ where
             let cascade_level = CascadeLevel::AuthorNormal {
                 shadow_cascade_order,
             };
-            collector.collect_rules_in_map(host_rules, cascade_level);
+            collector.collect_rules_in_map(host_rules, cascade_level, style_data);
         });
     }
 
@@ -386,30 +388,30 @@ where
 
             let inner_shadow_host = inner_shadow.host();
             let outer_shadow = inner_shadow_host.containing_shadow();
-            let part_rules = match outer_shadow {
-                Some(shadow) => shadow
-                    .style_data()
-                    .and_then(|data| data.part_rules(self.pseudo_element)),
-                None => self
+            let cascade_data = match outer_shadow {
+                Some(shadow) => shadow.style_data(),
+                None => Some(self
                     .stylist
                     .cascade_data()
                     .borrow_for_origin(Origin::Author)
-                    .part_rules(self.pseudo_element),
+                ),
             };
 
-            if let Some(part_rules) = part_rules {
-                let containing_host = outer_shadow.map(|s| s.host());
-                let cascade_level = CascadeLevel::AuthorNormal {
-                    shadow_cascade_order,
-                };
-                self.in_tree(containing_host, |collector| {
-                    for p in &parts {
-                        if let Some(part_rules) = part_rules.get(&p.0) {
-                            collector.collect_rules_in_list(part_rules, cascade_level);
+            if let Some(cascade_data) = cascade_data {
+                if let Some(part_rules) = cascade_data.part_rules(self.pseudo_element) {
+                    let containing_host = outer_shadow.map(|s| s.host());
+                    let cascade_level = CascadeLevel::AuthorNormal {
+                        shadow_cascade_order,
+                    };
+                    self.in_tree(containing_host, |collector| {
+                        for p in &parts {
+                            if let Some(part_rules) = part_rules.get(&p.0) {
+                                collector.collect_rules_in_list(part_rules, cascade_level, cascade_data);
+                            }
                         }
-                    }
-                });
-                shadow_cascade_order.inc();
+                    });
+                    shadow_cascade_order.inc();
+                }
             }
 
             inner_shadow = match outer_shadow {
diff --git a/components/style/selector_map.rs b/components/style/selector_map.rs
index 40806ed47af..a2b5d6bb6b6 100644
--- a/components/style/selector_map.rs
+++ b/components/style/selector_map.rs
@@ -12,7 +12,7 @@ use crate::hash::map as hash_map;
 use crate::hash::{HashMap, HashSet};
 use crate::rule_tree::CascadeLevel;
 use crate::selector_parser::SelectorImpl;
-use crate::stylist::Rule;
+use crate::stylist::{Rule, CascadeData};
 use crate::{Atom, LocalName, Namespace, WeakAtom};
 use fallible::FallibleVec;
 use hashglobe::FailedAllocationError;
@@ -104,10 +104,14 @@ pub struct SelectorMap<T: 'static> {
     pub class_hash: MaybeCaseInsensitiveHashMap<Atom, SmallVec<[T; 1]>>,
     /// A hash from local name to rules which contain that local name selector.
     pub local_name_hash: PrecomputedHashMap<LocalName, SmallVec<[T; 1]>>,
+    /// A hash from attributes to rules which contain that attribute selector.
+    pub attribute_hash: PrecomputedHashMap<LocalName, SmallVec<[T; 1]>>,
     /// A hash from namespace to rules which contain that namespace selector.
     pub namespace_hash: PrecomputedHashMap<Namespace, SmallVec<[T; 1]>>,
     /// All other rules.
     pub other: SmallVec<[T; 1]>,
+    /// Whether we should bucket by attribute names.
+    bucket_attributes: bool,
     /// The number of entries in this map.
     pub count: usize,
 }
@@ -129,18 +133,32 @@ impl<T: 'static> SelectorMap<T> {
             root: SmallVec::new(),
             id_hash: MaybeCaseInsensitiveHashMap::new(),
             class_hash: MaybeCaseInsensitiveHashMap::new(),
+            attribute_hash: HashMap::default(),
             local_name_hash: HashMap::default(),
             namespace_hash: HashMap::default(),
             other: SmallVec::new(),
+            #[cfg(feature = "gecko")]
+            bucket_attributes: static_prefs::pref!("layout.css.bucket-attribute-names.enabled"),
+            #[cfg(feature = "servo")]
+            bucket_attributes: false,
             count: 0,
         }
     }
 
+    /// Trivially constructs an empty `SelectorMap`, with attribute bucketing
+    /// explicitly disabled.
+    pub fn new_without_attribute_bucketing() -> Self {
+        let mut ret = Self::new();
+        ret.bucket_attributes = false;
+        ret
+    }
+
     /// Clears the hashmap retaining storage.
     pub fn clear(&mut self) {
         self.root.clear();
         self.id_hash.clear();
         self.class_hash.clear();
+        self.attribute_hash.clear();
         self.local_name_hash.clear();
         self.namespace_hash.clear();
         self.other.clear();
@@ -171,6 +189,7 @@ impl SelectorMap<Rule> {
         context: &mut MatchingContext<E::Impl>,
         flags_setter: &mut F,
         cascade_level: CascadeLevel,
+        cascade_data: &CascadeData,
     ) where
         E: TElement,
         F: FnMut(&E, ElementSelectorFlags),
@@ -189,6 +208,7 @@ impl SelectorMap<Rule> {
                 context,
                 flags_setter,
                 cascade_level,
+                cascade_data,
             );
         }
 
@@ -201,6 +221,7 @@ impl SelectorMap<Rule> {
                     context,
                     flags_setter,
                     cascade_level,
+                    cascade_data,
                 )
             }
         }
@@ -214,10 +235,27 @@ impl SelectorMap<Rule> {
                     context,
                     flags_setter,
                     cascade_level,
+                    cascade_data,
                 )
             }
         });
 
+        if self.bucket_attributes {
+            rule_hash_target.each_attr_name(|name| {
+                if let Some(rules) = self.attribute_hash.get(name) {
+                    SelectorMap::get_matching_rules(
+                        element,
+                        rules,
+                        matching_rules_list,
+                        context,
+                        flags_setter,
+                        cascade_level,
+                        cascade_data,
+                    )
+                }
+            });
+        }
+
         if let Some(rules) = self.local_name_hash.get(rule_hash_target.local_name()) {
             SelectorMap::get_matching_rules(
                 element,
@@ -226,6 +264,7 @@ impl SelectorMap<Rule> {
                 context,
                 flags_setter,
                 cascade_level,
+                cascade_data,
             )
         }
 
@@ -237,6 +276,7 @@ impl SelectorMap<Rule> {
                 context,
                 flags_setter,
                 cascade_level,
+                cascade_data,
             )
         }
 
@@ -247,6 +287,7 @@ impl SelectorMap<Rule> {
             context,
             flags_setter,
             cascade_level,
+            cascade_data,
         );
     }
 
@@ -258,6 +299,7 @@ impl SelectorMap<Rule> {
         context: &mut MatchingContext<E::Impl>,
         flags_setter: &mut F,
         cascade_level: CascadeLevel,
+        cascade_data: &CascadeData,
     ) where
         E: TElement,
         F: FnMut(&E, ElementSelectorFlags),
@@ -271,7 +313,7 @@ impl SelectorMap<Rule> {
                 context,
                 flags_setter,
             ) {
-                matching_rules.push(rule.to_applicable_declaration_block(cascade_level));
+                matching_rules.push(rule.to_applicable_declaration_block(cascade_level, cascade_data));
             }
         }
     }
@@ -302,6 +344,7 @@ impl<T: SelectorMapEntry> SelectorMap<T> {
                         .class_hash
                         .try_entry(class.clone(), quirks_mode)?
                         .or_insert_with(SmallVec::new),
+                    Bucket::Attribute { name, lower_name } |
                     Bucket::LocalName { name, lower_name } => {
                         // If the local name in the selector isn't lowercase,
                         // insert it into the rule hash twice. This means that,
@@ -316,13 +359,19 @@ impl<T: SelectorMapEntry> SelectorMap<T> {
                         // selector, the rulehash lookup may produce superfluous
                         // selectors, but the subsequent selector matching work
                         // will filter them out.
+                        let is_attribute = matches!($bucket, Bucket::Attribute { .. });
+                        let hash = if is_attribute {
+                            &mut self.attribute_hash
+                        } else {
+                            &mut self.local_name_hash
+                        };
                         if name != lower_name {
-                            self.local_name_hash
+                            hash
                                 .try_entry(lower_name.clone())?
                                 .or_insert_with(SmallVec::new)
                                 .try_push($entry.clone())?;
                         }
-                        self.local_name_hash
+                        hash
                             .try_entry(name.clone())?
                             .or_insert_with(SmallVec::new)
                     },
@@ -338,7 +387,7 @@ impl<T: SelectorMapEntry> SelectorMap<T> {
 
         let bucket = {
             let mut disjoint_buckets = SmallVec::new();
-            let bucket = find_bucket(entry.selector(), &mut disjoint_buckets);
+            let bucket = find_bucket(entry.selector(), &mut disjoint_buckets, self.bucket_attributes);
 
             // See if inserting this selector in multiple entries in the
             // selector map would be worth it. Consider a case like:
@@ -409,8 +458,29 @@ impl<T: SelectorMapEntry> SelectorMap<T> {
 
         let mut done = false;
         element.each_class(|class| {
-            if !done {
-                if let Some(v) = self.class_hash.get(class, quirks_mode) {
+            if done {
+                return;
+            }
+            if let Some(v) = self.class_hash.get(class, quirks_mode) {
+                for entry in v.iter() {
+                    if !f(&entry) {
+                        done = true;
+                        return;
+                    }
+                }
+            }
+        });
+
+        if done {
+            return false;
+        }
+
+        if self.bucket_attributes {
+            element.each_attr_name(|name| {
+                if done {
+                    return;
+                }
+                if let Some(v) = self.attribute_hash.get(name) {
                     for entry in v.iter() {
                         if !f(&entry) {
                             done = true;
@@ -418,10 +488,11 @@ impl<T: SelectorMapEntry> SelectorMap<T> {
                         }
                     }
                 }
+            });
+
+            if done {
+                return false;
             }
-        });
-        if done {
-            return false;
         }
 
         if let Some(v) = self.local_name_hash.get(element.local_name()) {
@@ -507,6 +578,10 @@ enum Bucket<'a> {
         name: &'a LocalName,
         lower_name: &'a LocalName,
     },
+    Attribute {
+        name: &'a LocalName,
+        lower_name: &'a LocalName,
+    },
     Class(&'a Atom),
     ID(&'a Atom),
     Root,
@@ -520,9 +595,10 @@ impl<'a> Bucket<'a> {
             Bucket::Universal => 0,
             Bucket::Namespace(..) => 1,
             Bucket::LocalName { .. } => 2,
-            Bucket::Class(..) => 3,
-            Bucket::ID(..) => 4,
-            Bucket::Root => 5,
+            Bucket::Attribute { .. } => 3,
+            Bucket::Class(..) => 4,
+            Bucket::ID(..) => 5,
+            Bucket::Root => 6,
         }
     }
 
@@ -537,11 +613,24 @@ type DisjointBuckets<'a> = SmallVec<[Bucket<'a>; 5]>;
 fn specific_bucket_for<'a>(
     component: &'a Component<SelectorImpl>,
     disjoint_buckets: &mut DisjointBuckets<'a>,
+    bucket_attributes: bool,
 ) -> Bucket<'a> {
     match *component {
         Component::Root => Bucket::Root,
         Component::ID(ref id) => Bucket::ID(id),
         Component::Class(ref class) => Bucket::Class(class),
+        Component::AttributeInNoNamespace { ref local_name, .. } if bucket_attributes => Bucket::Attribute {
+            name: local_name,
+            lower_name: local_name,
+        },
+        Component::AttributeInNoNamespaceExists { ref local_name, ref local_name_lower } if bucket_attributes => Bucket::Attribute {
+            name: local_name,
+            lower_name: local_name_lower,
+        },
+        Component::AttributeOther(ref selector) if bucket_attributes => Bucket::Attribute {
+            name: &selector.local_name,
+            lower_name: &selector.local_name_lower,
+        },
         Component::LocalName(ref selector) => Bucket::LocalName {
             name: &selector.name,
             lower_name: &selector.lower_name,
@@ -567,14 +656,14 @@ fn specific_bucket_for<'a>(
         //
         // So inserting `span` in the rule hash makes sense since we want to
         // match the slotted <span>.
-        Component::Slotted(ref selector) => find_bucket(selector.iter(), disjoint_buckets),
-        Component::Host(Some(ref selector)) => find_bucket(selector.iter(), disjoint_buckets),
+        Component::Slotted(ref selector) => find_bucket(selector.iter(), disjoint_buckets, bucket_attributes),
+        Component::Host(Some(ref selector)) => find_bucket(selector.iter(), disjoint_buckets, bucket_attributes),
         Component::Is(ref list) | Component::Where(ref list) => {
             if list.len() == 1 {
-                find_bucket(list[0].iter(), disjoint_buckets)
+                find_bucket(list[0].iter(), disjoint_buckets, bucket_attributes)
             } else {
                 for selector in &**list {
-                    let bucket = find_bucket(selector.iter(), disjoint_buckets);
+                    let bucket = find_bucket(selector.iter(), disjoint_buckets, bucket_attributes);
                     disjoint_buckets.push(bucket);
                 }
                 Bucket::Universal
@@ -593,12 +682,13 @@ fn specific_bucket_for<'a>(
 fn find_bucket<'a>(
     mut iter: SelectorIter<'a, SelectorImpl>,
     disjoint_buckets: &mut DisjointBuckets<'a>,
+    bucket_attributes: bool,
 ) -> Bucket<'a> {
     let mut current_bucket = Bucket::Universal;
 
     loop {
         for ss in &mut iter {
-            let new_bucket = specific_bucket_for(ss, disjoint_buckets);
+            let new_bucket = specific_bucket_for(ss, disjoint_buckets, bucket_attributes);
             if new_bucket.more_specific_than(&current_bucket) {
                 current_bucket = new_bucket;
             }
diff --git a/components/style/servo/media_queries.rs b/components/style/servo/media_queries.rs
index cee76053ec0..d575b7c2df7 100644
--- a/components/style/servo/media_queries.rs
+++ b/components/style/servo/media_queries.rs
@@ -190,12 +190,12 @@ impl Device {
     }
 
     /// Returns the default background color.
-    pub fn default_background_color(&self) -> RGBA {
+    pub fn default_background_color_for_forced_colors(&self) -> RGBA {
         RGBA::new(255, 255, 255, 255)
     }
 
-    /// Returns the default color color.
-    pub fn default_color(&self) -> RGBA {
+    /// Returns the default foreground color.
+    pub fn default_color_for_forced_colors(&self) -> RGBA {
         RGBA::new(0, 0, 0, 255)
     }
 
@@ -220,6 +220,24 @@ impl Device {
             _ => false,
         }
     }
+
+    /// Returns the gtk titlebar radius in CSS pixels.
+    /// TODO: implement this method.
+    pub fn titlebar_radius(&self) -> f32 {
+        0.0
+    }
+
+    /// Returns the gtk menu radius in CSS pixels.
+    /// TODO: implement this method.
+    pub fn menu_radius(&self) -> f32 {
+        0.0
+    }
+
+    /// Return whether the document is a chrome document.
+    #[inline]
+    pub fn is_chrome_document(&self) -> bool {
+        false
+    }
 }
 
 /// https://drafts.csswg.org/mediaqueries-4/#width
diff --git a/components/style/shared_lock.rs b/components/style/shared_lock.rs
index 5bfa7d19737..d524f0c6cdf 100644
--- a/components/style/shared_lock.rs
+++ b/components/style/shared_lock.rs
@@ -94,6 +94,12 @@ impl SharedRwLock {
         SharedRwLock { cell: None }
     }
 
+    #[cfg(feature = "gecko")]
+    #[inline]
+    fn ptr(&self) -> *const SomethingZeroSizedButTyped {
+        self.cell.as_ref().map(|cell| cell.as_ptr() as *const _).unwrap_or(ptr::null())
+    }
+
     /// Wrap the given data to make its access protected by this lock.
     pub fn wrap<T>(&self, data: T) -> Locked<T> {
         Locked {
@@ -144,6 +150,14 @@ impl<'a> Drop for SharedRwLockReadGuard<'a> {
     }
 }
 
+impl<'a> SharedRwLockReadGuard<'a> {
+    #[inline]
+    #[cfg(feature = "gecko")]
+    fn ptr(&self) -> *const SomethingZeroSizedButTyped {
+        self.0.as_ref().map(|r| &**r as *const _).unwrap_or(ptr::null())
+    }
+}
+
 /// Proof that a shared lock was obtained for writing (servo).
 #[cfg(feature = "servo")]
 pub struct SharedRwLockWriteGuard<'a>(&'a SharedRwLock);
@@ -190,25 +204,18 @@ impl<T> Locked<T> {
     }
 
     #[cfg(feature = "gecko")]
-    fn same_lock_as(&self, derefed_guard: Option<&SomethingZeroSizedButTyped>) -> bool {
-        ptr::eq(
-            self.shared_lock
-                .cell
-                .as_ref()
-                .map(|cell| cell.as_ptr())
-                .unwrap_or(ptr::null_mut()),
-            derefed_guard
-                .map(|guard| guard as *const _ as *mut _)
-                .unwrap_or(ptr::null_mut()),
-        )
+    fn same_lock_as(&self, ptr: *const SomethingZeroSizedButTyped) -> bool {
+        ptr::eq(self.shared_lock.ptr(), ptr)
     }
 
     /// Access the data for reading.
     pub fn read_with<'a>(&'a self, guard: &'a SharedRwLockReadGuard) -> &'a T {
         #[cfg(feature = "gecko")]
         assert!(
-            self.is_read_only_lock() || self.same_lock_as(guard.0.as_ref().map(|r| &**r)),
-            "Locked::read_with called with a guard from an unrelated SharedRwLock"
+            self.is_read_only_lock() || self.same_lock_as(guard.ptr()),
+            "Locked::read_with called with a guard from an unrelated SharedRwLock: {:?} vs. {:?}",
+            self.shared_lock.ptr(),
+            guard.ptr(),
         );
         #[cfg(not(feature = "gecko"))]
         assert!(self.same_lock_as(&guard.0));
@@ -235,7 +242,7 @@ impl<T> Locked<T> {
     pub fn write_with<'a>(&'a self, guard: &'a mut SharedRwLockWriteGuard) -> &'a mut T {
         #[cfg(feature = "gecko")]
         assert!(
-            !self.is_read_only_lock() && self.same_lock_as(Some(&guard.0)),
+            !self.is_read_only_lock() && self.same_lock_as(&*guard.0),
             "Locked::write_with called with a guard from a read only or unrelated SharedRwLock"
         );
         #[cfg(not(feature = "gecko"))]
diff --git a/components/style/sharing/mod.rs b/components/style/sharing/mod.rs
index 49b34fbbbff..50eb51fba35 100644
--- a/components/style/sharing/mod.rs
+++ b/components/style/sharing/mod.rs
@@ -786,10 +786,7 @@ impl<E: TElement> StyleSharingCache<E> {
         }
 
         // It's possible that there are no styles for either id.
-        let may_match_different_id_rules =
-            checks::may_match_different_id_rules(shared, target.element, candidate.element);
-
-        if may_match_different_id_rules {
+        if checks::may_match_different_id_rules(shared, target.element, candidate.element) {
             trace!("Miss: ID Attr");
             return None;
         }
diff --git a/components/style/style_adjuster.rs b/components/style/style_adjuster.rs
index 5e66e91e894..31028f98571 100644
--- a/components/style/style_adjuster.rs
+++ b/components/style/style_adjuster.rs
@@ -152,63 +152,6 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
         }
     }
 
-    /// https://html.spec.whatwg.org/multipage/#inert-subtrees
-    ///
-    ///    If -moz-inert is applied then add:
-    ///        -moz-user-focus: none;
-    ///        -moz-user-input: none;
-    ///        -moz-user-modify: read-only;
-    ///        user-select: none;
-    ///        pointer-events: none;
-    ///        cursor: default;
-    ///
-    /// NOTE: dialog:-moz-topmost-modal-dialog is used to override above
-    /// rules to remove the inertness for the topmost modal dialog.
-    ///
-    /// NOTE: If this or the pointer-events tweak is removed, then
-    /// minimal-xul.css and the scrollbar style caching need to be tweaked.
-    #[cfg(feature = "gecko")]
-    fn adjust_for_inert(&mut self) {
-        use crate::values::specified::ui::CursorKind;
-        use crate::values::specified::ui::UserSelect;
-        use properties::longhands::_moz_inert::computed_value::T as Inert;
-        use properties::longhands::_moz_user_focus::computed_value::T as UserFocus;
-        use properties::longhands::_moz_user_input::computed_value::T as UserInput;
-        use properties::longhands::_moz_user_modify::computed_value::T as UserModify;
-        use properties::longhands::cursor::computed_value::T as Cursor;
-        use properties::longhands::pointer_events::computed_value::T as PointerEvents;
-
-        let needs_update = {
-            let ui = self.style.get_inherited_ui();
-            if ui.clone__moz_inert() == Inert::None {
-                return;
-            }
-
-            ui.clone__moz_user_focus() != UserFocus::None ||
-                ui.clone__moz_user_input() != UserInput::None ||
-                ui.clone__moz_user_modify() != UserModify::ReadOnly ||
-                ui.clone_pointer_events() != PointerEvents::None ||
-                ui.clone_cursor().keyword != CursorKind::Default ||
-                ui.clone_cursor().images != Default::default()
-        };
-
-        if needs_update {
-            let ui = self.style.mutate_inherited_ui();
-            ui.set__moz_user_focus(UserFocus::None);
-            ui.set__moz_user_input(UserInput::None);
-            ui.set__moz_user_modify(UserModify::ReadOnly);
-            ui.set_pointer_events(PointerEvents::None);
-            ui.set_cursor(Cursor {
-                images: Default::default(),
-                keyword: CursorKind::Default,
-            });
-        }
-
-        if self.style.get_ui().clone_user_select() != UserSelect::None {
-            self.style.mutate_ui().set_user_select(UserSelect::None);
-        }
-    }
-
     /// Whether we should skip any item-based display property blockification on
     /// this element.
     fn skip_item_display_fixup<E>(&self, element: Option<E>) -> bool
@@ -909,7 +852,6 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
         #[cfg(feature = "gecko")]
         {
             self.adjust_for_appearance(element);
-            self.adjust_for_inert();
             self.adjust_for_marker_pseudo();
         }
         self.set_bits();
diff --git a/components/style/stylesheets/font_feature_values_rule.rs b/components/style/stylesheets/font_feature_values_rule.rs
index dc9e9f333db..dc128e77fb0 100644
--- a/components/style/stylesheets/font_feature_values_rule.rs
+++ b/components/style/stylesheets/font_feature_values_rule.rs
@@ -396,13 +396,14 @@ macro_rules! font_feature_values_blocks {
             type AtRule = ();
             type Error = StyleParseErrorKind<'i>;
 
-            fn parse_prelude<'t>(&mut self,
-                                 name: CowRcStr<'i>,
-                                 input: &mut Parser<'i, 't>)
-                                 -> Result<Self::Prelude, ParseError<'i>> {
+            fn parse_prelude<'t>(
+                &mut self,
+                name: CowRcStr<'i>,
+                input: &mut Parser<'i, 't>,
+            ) -> Result<BlockType, ParseError<'i>> {
                 match_ignore_ascii_case! { &*name,
                     $(
-                        $name => Ok(Self::Prelude::$ident_camel),
+                        $name => Ok(BlockType::$ident_camel),
                     )*
                     _ => Err(input.new_error(BasicParseErrorKind::AtRuleBodyInvalid)),
                 }
diff --git a/components/style/stylesheets/import_rule.rs b/components/style/stylesheets/import_rule.rs
index 8f518f29bac..7352dea0b9e 100644
--- a/components/style/stylesheets/import_rule.rs
+++ b/components/style/stylesheets/import_rule.rs
@@ -11,21 +11,13 @@ use crate::shared_lock::{DeepCloneParams, DeepCloneWithLock};
 use crate::shared_lock::{SharedRwLock, SharedRwLockReadGuard, ToCssWithGuard};
 use crate::str::CssStringWriter;
 use crate::stylesheets::{CssRule, StylesheetInDocument};
+use crate::stylesheets::layer_rule::LayerName;
 use crate::values::CssUrl;
 use cssparser::SourceLocation;
 use std::fmt::{self, Write};
 use style_traits::{CssWriter, ToCss};
 use to_shmem::{self, SharedMemoryBuilder, ToShmem};
 
-/// With asynchronous stylesheet parsing, we can't synchronously create a
-/// GeckoStyleSheet. So we use this placeholder instead.
-#[cfg(feature = "gecko")]
-#[derive(Clone, Debug)]
-pub struct PendingSheet {
-    origin: Origin,
-    quirks_mode: QuirksMode,
-}
-
 /// A sheet that is held from an import rule.
 #[cfg(feature = "gecko")]
 #[derive(Debug)]
@@ -34,7 +26,7 @@ pub enum ImportSheet {
     Sheet(crate::gecko::data::GeckoStyleSheet),
     /// An @import created while parsing off-main-thread, whose Gecko sheet has
     /// yet to be created and attached.
-    Pending(PendingSheet),
+    Pending,
 }
 
 #[cfg(feature = "gecko")]
@@ -45,11 +37,8 @@ impl ImportSheet {
     }
 
     /// Creates a pending ImportSheet for a load that has not started yet.
-    pub fn new_pending(origin: Origin, quirks_mode: QuirksMode) -> Self {
-        ImportSheet::Pending(PendingSheet {
-            origin,
-            quirks_mode,
-        })
+    pub fn new_pending() -> Self {
+        ImportSheet::Pending
     }
 
     /// Returns a reference to the GeckoStyleSheet in this ImportSheet, if it
@@ -63,7 +52,7 @@ impl ImportSheet {
                 }
                 Some(s)
             },
-            ImportSheet::Pending(_) => None,
+            ImportSheet::Pending => None,
         }
     }
 
@@ -98,7 +87,7 @@ impl DeepCloneWithLock for ImportSheet {
                 };
                 ImportSheet::Sheet(unsafe { GeckoStyleSheet::from_addrefed(clone) })
             },
-            ImportSheet::Pending(ref p) => ImportSheet::Pending(p.clone()),
+            ImportSheet::Pending => ImportSheet::Pending,
         }
     }
 }
@@ -135,6 +124,30 @@ impl DeepCloneWithLock for ImportSheet {
     }
 }
 
+/// The layer keyword or function in an import rule.
+#[derive(Debug, Clone)]
+pub struct ImportLayer {
+    /// The layer name, or None for an anonymous layer.
+    pub name: Option<LayerName>,
+}
+
+
+impl ToCss for ImportLayer {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        match self.name {
+            None => dest.write_str("layer"),
+            Some(ref name) => {
+                dest.write_str("layer(")?;
+                name.to_css(dest)?;
+                dest.write_char(')')
+            },
+        }
+    }
+}
+
 /// The [`@import`][import] at-rule.
 ///
 /// [import]: https://drafts.csswg.org/css-cascade-3/#at-import
@@ -148,6 +161,9 @@ pub struct ImportRule {
     /// ImportSheet just has stub behavior until it appears.
     pub stylesheet: ImportSheet,
 
+    /// A `layer()` function name.
+    pub layer: Option<ImportLayer>,
+
     /// The line and column of the rule's source code.
     pub source_location: SourceLocation,
 }
@@ -170,6 +186,7 @@ impl DeepCloneWithLock for ImportRule {
         ImportRule {
             url: self.url.clone(),
             stylesheet: self.stylesheet.deep_clone_with_lock(lock, guard, params),
+            layer: self.layer.clone(),
             source_location: self.source_location.clone(),
         }
     }
@@ -180,14 +197,18 @@ impl ToCssWithGuard for ImportRule {
         dest.write_str("@import ")?;
         self.url.to_css(&mut CssWriter::new(dest))?;
 
-        match self.stylesheet.media(guard) {
-            Some(media) if !media.is_empty() => {
-                dest.write_str(" ")?;
+        if let Some(media) = self.stylesheet.media(guard) {
+            if !media.is_empty() {
+                dest.write_char(' ')?;
                 media.to_css(&mut CssWriter::new(dest))?;
-            },
-            _ => {},
-        };
+            }
+        }
 
-        dest.write_str(";")
+        if let Some(ref layer) = self.layer {
+            dest.write_char(' ')?;
+            layer.to_css(&mut CssWriter::new(dest))?;
+        }
+
+        dest.write_char(';')
     }
 }
diff --git a/components/style/stylesheets/keyframes_rule.rs b/components/style/stylesheets/keyframes_rule.rs
index 374f6f1d8ec..8b721b7a52c 100644
--- a/components/style/stylesheets/keyframes_rule.rs
+++ b/components/style/stylesheets/keyframes_rule.rs
@@ -14,6 +14,7 @@ use crate::properties::{PropertyDeclarationId, SourcePropertyDeclaration};
 use crate::shared_lock::{DeepCloneParams, DeepCloneWithLock, SharedRwLock, SharedRwLockReadGuard};
 use crate::shared_lock::{Locked, ToCssWithGuard};
 use crate::str::CssStringWriter;
+use crate::stylesheets::layer_rule::LayerId;
 use crate::stylesheets::rule_parser::VendorPrefix;
 use crate::stylesheets::{CssRuleType, StylesheetContents};
 use crate::values::{serialize_percentage, KeyframesName};
@@ -357,6 +358,8 @@ pub struct KeyframesAnimation {
     pub properties_changed: LonghandIdSet,
     /// Vendor prefix type the @keyframes has.
     pub vendor_prefix: Option<VendorPrefix>,
+    /// The id of the cascade layer the keyframe rule was in.
+    pub layer_id: LayerId,
 }
 
 /// Get all the animated properties in a keyframes animation.
@@ -409,12 +412,14 @@ impl KeyframesAnimation {
     pub fn from_keyframes(
         keyframes: &[Arc<Locked<Keyframe>>],
         vendor_prefix: Option<VendorPrefix>,
+        layer_id: LayerId,
         guard: &SharedRwLockReadGuard,
     ) -> Self {
         let mut result = KeyframesAnimation {
             steps: vec![],
             properties_changed: LonghandIdSet::new(),
             vendor_prefix,
+            layer_id,
         };
 
         if keyframes.is_empty() {
diff --git a/components/style/stylesheets/layer_rule.rs b/components/style/stylesheets/layer_rule.rs
new file mode 100644
index 00000000000..ee066d813e2
--- /dev/null
+++ b/components/style/stylesheets/layer_rule.rs
@@ -0,0 +1,235 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! A [`@layer`][layer] urle.
+//!
+//! [layer]: https://drafts.csswg.org/css-cascade-5/#layering
+
+use crate::parser::{Parse, ParserContext};
+use crate::shared_lock::{DeepCloneParams, DeepCloneWithLock, Locked};
+use crate::shared_lock::{SharedRwLock, SharedRwLockReadGuard, ToCssWithGuard};
+use crate::values::AtomIdent;
+
+use super::CssRules;
+
+use cssparser::{Parser, SourceLocation, ToCss as CssParserToCss, Token};
+use servo_arc::Arc;
+use smallvec::SmallVec;
+use std::fmt::{self, Write};
+use style_traits::{CssWriter, ParseError, ToCss};
+
+/// The order of a given layer.
+#[derive(Clone, Copy, Debug, Eq, MallocSizeOf, PartialEq, PartialOrd, Ord)]
+pub struct LayerOrder(u32);
+
+impl LayerOrder {
+    /// The order of the root layer.
+    pub const fn root() -> Self {
+        Self(std::u32::MAX)
+    }
+
+    /// The first cascade layer order.
+    pub const fn first() -> Self {
+        Self(0)
+    }
+
+    /// Increment the cascade layer order.
+    #[inline]
+    pub fn inc(&mut self) {
+        self.0 += 1;
+    }
+}
+
+/// The id of a given layer, a sequentially-increasing identifier.
+#[derive(Clone, Copy, Debug, Eq, MallocSizeOf, PartialEq, PartialOrd, Ord)]
+pub struct LayerId(pub u32);
+
+impl LayerId {
+    /// The id of the root layer.
+    pub const fn root() -> Self {
+        Self(0)
+    }
+}
+
+/// A `<layer-name>`: https://drafts.csswg.org/css-cascade-5/#typedef-layer-name
+#[derive(Clone, Debug, Eq, Hash, MallocSizeOf, PartialEq, ToShmem)]
+pub struct LayerName(pub SmallVec<[AtomIdent; 1]>);
+
+impl LayerName {
+    /// Returns an empty layer name (which isn't a valid final state, so caller
+    /// is responsible to fill up the name before use).
+    pub fn new_empty() -> Self {
+        Self(Default::default())
+    }
+
+    /// Returns a synthesized name for an anonymous layer.
+    pub fn new_anonymous() -> Self {
+        use std::sync::atomic::{AtomicUsize, Ordering};
+        static NEXT_ANONYMOUS_LAYER_NAME: AtomicUsize = AtomicUsize::new(0);
+
+        let mut name = SmallVec::new();
+        let next_id = NEXT_ANONYMOUS_LAYER_NAME.fetch_add(1, Ordering::Relaxed);
+        // The parens don't _technically_ prevent conflicts with authors, as
+        // authors could write escaped parens as part of the identifier, I
+        // think, but highly reduces the possibility.
+        name.push(AtomIdent::from(&*format!("-moz-anon-layer({})", next_id)));
+
+        LayerName(name)
+    }
+
+    /// Returns the names of the layers. That is, for a layer like `foo.bar`,
+    /// it'd return [foo, bar].
+    pub fn layer_names(&self) -> &[AtomIdent] {
+        &self.0
+    }
+}
+
+impl Parse for LayerName {
+    fn parse<'i, 't>(
+        _: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        let mut result = SmallVec::new();
+        result.push(AtomIdent::from(&**input.expect_ident()?));
+        loop {
+            let next_name = input.try_parse(|input| -> Result<AtomIdent, ParseError<'i>> {
+                match input.next_including_whitespace()? {
+                    Token::Delim('.') => {},
+                    other => {
+                        let t = other.clone();
+                        return Err(input.new_unexpected_token_error(t));
+                    },
+                }
+
+                let name = match input.next_including_whitespace()? {
+                    Token::Ident(ref ident) => ident,
+                    other => {
+                        let t = other.clone();
+                        return Err(input.new_unexpected_token_error(t));
+                    },
+                };
+
+                Ok(AtomIdent::from(&**name))
+            });
+
+            match next_name {
+                Ok(name) => result.push(name),
+                Err(..) => break,
+            }
+        }
+        Ok(LayerName(result))
+    }
+}
+
+impl ToCss for LayerName {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        let mut first = true;
+        for name in self.0.iter() {
+            if !first {
+                dest.write_char('.')?;
+            }
+            first = false;
+            name.to_css(dest)?;
+        }
+        Ok(())
+    }
+}
+
+/// The kind of layer rule this is.
+#[derive(Debug, ToShmem)]
+pub enum LayerRuleKind {
+    /// A block `@layer <name>? { ... }`
+    Block {
+        /// The layer name, or `None` if anonymous.
+        name: Option<LayerName>,
+        /// The nested rules.
+        rules: Arc<Locked<CssRules>>,
+    },
+    /// A statement `@layer <name>, <name>, <name>;`
+    Statement {
+        /// The list of layers to sort.
+        names: Vec<LayerName>,
+    },
+}
+
+/// A [`@layer`][layer] rule.
+///
+/// [layer]: https://drafts.csswg.org/css-cascade-5/#layering
+#[derive(Debug, ToShmem)]
+pub struct LayerRule {
+    /// The kind of layer rule we are.
+    pub kind: LayerRuleKind,
+    /// The source position where this media rule was found.
+    pub source_location: SourceLocation,
+}
+
+impl ToCssWithGuard for LayerRule {
+    fn to_css(
+        &self,
+        guard: &SharedRwLockReadGuard,
+        dest: &mut crate::str::CssStringWriter,
+    ) -> fmt::Result {
+        dest.write_str("@layer")?;
+        match self.kind {
+            LayerRuleKind::Block {
+                ref name,
+                ref rules,
+            } => {
+                if let Some(ref name) = *name {
+                    dest.write_char(' ')?;
+                    name.to_css(&mut CssWriter::new(dest))?;
+                }
+                rules.read_with(guard).to_css_block(guard, dest)
+            },
+            LayerRuleKind::Statement { ref names } => {
+                let mut writer = CssWriter::new(dest);
+                let mut first = true;
+                for name in &**names {
+                    if first {
+                        writer.write_char(' ')?;
+                    } else {
+                        writer.write_str(", ")?;
+                    }
+                    first = false;
+                    name.to_css(&mut writer)?;
+                }
+                dest.write_char(';')
+            },
+        }
+    }
+}
+
+impl DeepCloneWithLock for LayerRule {
+    fn deep_clone_with_lock(
+        &self,
+        lock: &SharedRwLock,
+        guard: &SharedRwLockReadGuard,
+        params: &DeepCloneParams,
+    ) -> Self {
+        Self {
+            kind: match self.kind {
+                LayerRuleKind::Block {
+                    ref name,
+                    ref rules,
+                } => LayerRuleKind::Block {
+                    name: name.clone(),
+                    rules: Arc::new(
+                        lock.wrap(
+                            rules
+                                .read_with(guard)
+                                .deep_clone_with_lock(lock, guard, params),
+                        ),
+                    ),
+                },
+                LayerRuleKind::Statement { ref names } => LayerRuleKind::Statement {
+                    names: names.clone(),
+                },
+            },
+            source_location: self.source_location.clone(),
+        }
+    }
+}
diff --git a/components/style/stylesheets/loader.rs b/components/style/stylesheets/loader.rs
index ce7c563db05..c145bafdc04 100644
--- a/components/style/stylesheets/loader.rs
+++ b/components/style/stylesheets/loader.rs
@@ -8,7 +8,7 @@
 use crate::media_queries::MediaList;
 use crate::parser::ParserContext;
 use crate::shared_lock::{Locked, SharedRwLock};
-use crate::stylesheets::import_rule::ImportRule;
+use crate::stylesheets::import_rule::{ImportRule, ImportLayer};
 use crate::values::CssUrl;
 use cssparser::SourceLocation;
 use servo_arc::Arc;
@@ -25,5 +25,6 @@ pub trait StylesheetLoader {
         context: &ParserContext,
         lock: &SharedRwLock,
         media: Arc<Locked<MediaList>>,
+        layer: Option<ImportLayer>,
     ) -> Arc<Locked<ImportRule>>;
 }
diff --git a/components/style/stylesheets/mod.rs b/components/style/stylesheets/mod.rs
index fd9be56b5b5..762bca6033f 100644
--- a/components/style/stylesheets/mod.rs
+++ b/components/style/stylesheets/mod.rs
@@ -11,6 +11,7 @@ mod font_face_rule;
 pub mod font_feature_values_rule;
 pub mod import_rule;
 pub mod keyframes_rule;
+pub mod layer_rule;
 mod loader;
 mod media_rule;
 mod namespace_rule;
@@ -19,6 +20,7 @@ mod page_rule;
 mod rule_list;
 mod rule_parser;
 mod rules_iterator;
+pub mod scroll_timeline_rule;
 mod style_rule;
 mod stylesheet;
 pub mod supports_rule;
@@ -49,6 +51,7 @@ pub use self::font_face_rule::FontFaceRule;
 pub use self::font_feature_values_rule::FontFeatureValuesRule;
 pub use self::import_rule::ImportRule;
 pub use self::keyframes_rule::KeyframesRule;
+pub use self::layer_rule::LayerRule;
 pub use self::loader::StylesheetLoader;
 pub use self::media_rule::MediaRule;
 pub use self::namespace_rule::NamespaceRule;
@@ -60,6 +63,7 @@ pub use self::rules_iterator::{AllRules, EffectiveRules};
 pub use self::rules_iterator::{
     EffectiveRulesIterator, NestedRuleIterationCondition, RulesIterator,
 };
+pub use self::scroll_timeline_rule::ScrollTimelineRule;
 pub use self::style_rule::StyleRule;
 pub use self::stylesheet::{AllowImportRules, SanitizationData, SanitizationKind};
 pub use self::stylesheet::{DocumentStyleSheet, Namespaces, Stylesheet};
@@ -257,6 +261,8 @@ pub enum CssRule {
     Supports(Arc<Locked<SupportsRule>>),
     Page(Arc<Locked<PageRule>>),
     Document(Arc<Locked<DocumentRule>>),
+    Layer(Arc<Locked<LayerRule>>),
+    ScrollTimeline(Arc<Locked<ScrollTimelineRule>>),
 }
 
 impl CssRule {
@@ -297,16 +303,22 @@ impl CssRule {
             CssRule::Document(ref lock) => {
                 lock.unconditional_shallow_size_of(ops) + lock.read_with(guard).size_of(guard, ops)
             },
+
+            // TODO(emilio): Add memory reporting for @layer rules.
+            CssRule::Layer(_) => 0,
+            CssRule::ScrollTimeline(_) => 0,
         }
     }
 }
 
+/// https://drafts.csswg.org/cssom-1/#dom-cssrule-type
 #[allow(missing_docs)]
 #[derive(Clone, Copy, Debug, Eq, FromPrimitive, PartialEq)]
+#[repr(u8)]
 pub enum CssRuleType {
     // https://drafts.csswg.org/cssom/#the-cssrule-interface
     Style = 1,
-    Charset = 2,
+    // Charset = 2, // Historical
     Import = 3,
     Media = 4,
     FontFace = 5,
@@ -315,7 +327,7 @@ pub enum CssRuleType {
     Keyframes = 7,
     Keyframe = 8,
     // https://drafts.csswg.org/cssom/#the-cssrule-interface
-    Margin = 9,
+    // Margin = 9, // Not implemented yet.
     Namespace = 10,
     // https://drafts.csswg.org/css-counter-styles-3/#extentions-to-cssrule-interface
     CounterStyle = 11,
@@ -323,10 +335,14 @@ pub enum CssRuleType {
     Supports = 12,
     // https://www.w3.org/TR/2012/WD-css3-conditional-20120911/#extentions-to-cssrule-interface
     Document = 13,
-    // https://drafts.csswg.org/css-fonts-3/#om-fontfeaturevalues
+    // https://drafts.csswg.org/css-fonts/#om-fontfeaturevalues
     FontFeatureValues = 14,
     // https://drafts.csswg.org/css-device-adapt/#css-rule-interface
     Viewport = 15,
+    // After viewport, all rules should return 0 from the API, but we still need
+    // a constant somewhere.
+    Layer = 16,
+    ScrollTimeline = 17,
 }
 
 #[allow(missing_docs)]
@@ -353,6 +369,8 @@ impl CssRule {
             CssRule::Supports(_) => CssRuleType::Supports,
             CssRule::Page(_) => CssRuleType::Page,
             CssRule::Document(_) => CssRuleType::Document,
+            CssRule::Layer(_) => CssRuleType::Layer,
+            CssRule::ScrollTimeline(_) => CssRuleType::ScrollTimeline,
         }
     }
 
@@ -361,6 +379,7 @@ impl CssRule {
             // CssRule::Charset(..) => State::Start,
             CssRule::Import(..) => State::Imports,
             CssRule::Namespace(..) => State::Namespaces,
+            // TODO(emilio): Do we need something for EarlyLayers?
             _ => State::Body,
         }
     }
@@ -485,6 +504,16 @@ impl DeepCloneWithLock for CssRule {
                     lock.wrap(rule.deep_clone_with_lock(lock, guard, params)),
                 ))
             },
+            CssRule::Layer(ref arc) => {
+                let rule = arc.read_with(guard);
+                CssRule::Layer(Arc::new(
+                    lock.wrap(rule.deep_clone_with_lock(lock, guard, params)),
+                ))
+            }
+            CssRule::ScrollTimeline(ref arc) => {
+                let rule = arc.read_with(guard);
+                CssRule::ScrollTimeline(Arc::new(lock.wrap(rule.clone())))
+            }
         }
     }
 }
@@ -505,6 +534,8 @@ impl ToCssWithGuard for CssRule {
             CssRule::Supports(ref lock) => lock.read_with(guard).to_css(guard, dest),
             CssRule::Page(ref lock) => lock.read_with(guard).to_css(guard, dest),
             CssRule::Document(ref lock) => lock.read_with(guard).to_css(guard, dest),
+            CssRule::Layer(ref lock) => lock.read_with(guard).to_css(guard, dest),
+            CssRule::ScrollTimeline(ref lock) => lock.read_with(guard).to_css(guard, dest),
         }
     }
 }
diff --git a/components/style/stylesheets/rule_parser.rs b/components/style/stylesheets/rule_parser.rs
index 914af2cfb66..7e3e4f9f86c 100644
--- a/components/style/stylesheets/rule_parser.rs
+++ b/components/style/stylesheets/rule_parser.rs
@@ -15,20 +15,23 @@ use crate::shared_lock::{Locked, SharedRwLock};
 use crate::str::starts_with_ignore_ascii_case;
 use crate::stylesheets::document_rule::DocumentCondition;
 use crate::stylesheets::font_feature_values_rule::parse_family_name_list;
+use crate::stylesheets::import_rule::ImportLayer;
 use crate::stylesheets::keyframes_rule::parse_keyframe_list;
+use crate::stylesheets::layer_rule::{LayerName, LayerRuleKind};
+use crate::stylesheets::scroll_timeline_rule::ScrollTimelineDescriptors;
 use crate::stylesheets::stylesheet::Namespaces;
 use crate::stylesheets::supports_rule::SupportsCondition;
-use crate::stylesheets::viewport_rule;
-use crate::stylesheets::AllowImportRules;
-use crate::stylesheets::{CorsMode, DocumentRule, FontFeatureValuesRule, KeyframesRule, MediaRule};
-use crate::stylesheets::{CssRule, CssRuleType, CssRules, RulesMutateError, StylesheetLoader};
-use crate::stylesheets::{NamespaceRule, PageRule, StyleRule, SupportsRule, ViewportRule};
+use crate::stylesheets::{
+    viewport_rule, AllowImportRules, CorsMode, CssRule, CssRuleType, CssRules, DocumentRule,
+    FontFeatureValuesRule, KeyframesRule, LayerRule, MediaRule, NamespaceRule, PageRule,
+    RulesMutateError, ScrollTimelineRule, StyleRule, StylesheetLoader, SupportsRule, ViewportRule,
+};
 use crate::values::computed::font::FamilyName;
-use crate::values::{CssUrl, CustomIdent, KeyframesName};
+use crate::values::{CssUrl, CustomIdent, KeyframesName, TimelineName};
 use crate::{Namespace, Prefix};
-use cssparser::{AtRuleParser, Parser, QualifiedRuleParser, RuleListParser};
 use cssparser::{
-    BasicParseError, BasicParseErrorKind, CowRcStr, ParseErrorKind, ParserState, SourcePosition,
+    AtRuleParser, BasicParseError, BasicParseErrorKind, CowRcStr, Parser, ParserState,
+    QualifiedRuleParser, RuleListParser, SourcePosition,
 };
 use selectors::SelectorList;
 use servo_arc::Arc;
@@ -130,12 +133,14 @@ impl<'b> TopLevelRuleParser<'b> {
 pub enum State {
     /// We haven't started parsing rules.
     Start = 1,
-    /// We're parsing `@import` rules.
-    Imports = 2,
+    /// We're parsing early `@layer` statement rules.
+    EarlyLayers = 2,
+    /// We're parsing `@import` and early `@layer` statement rules.
+    Imports = 3,
     /// We're parsing `@namespace` rules.
-    Namespaces = 3,
+    Namespaces = 4,
     /// We're parsing the main body of the stylesheet.
-    Body = 4,
+    Body = 5,
 }
 
 #[derive(Clone, Debug, MallocSizeOf, ToShmem)]
@@ -168,9 +173,13 @@ pub enum AtRulePrelude {
     /// A @document rule, with its conditional.
     Document(DocumentCondition),
     /// A @import rule prelude.
-    Import(CssUrl, Arc<Locked<MediaList>>),
+    Import(CssUrl, Arc<Locked<MediaList>>, Option<ImportLayer>),
     /// A @namespace rule prelude.
     Namespace(Option<Prefix>, Namespace),
+    /// A @layer rule prelude.
+    Layer(Vec<LayerName>),
+    /// A @scroll-timeline rule prelude.
+    ScrollTimeline(TimelineName),
 }
 
 impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
@@ -182,7 +191,7 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
         &mut self,
         name: CowRcStr<'i>,
         input: &mut Parser<'i, 't>,
-    ) -> Result<Self::Prelude, ParseError<'i>> {
+    ) -> Result<AtRulePrelude, ParseError<'i>> {
         match_ignore_ascii_case! { &*name,
             "import" => {
                 if !self.check_state(State::Imports) {
@@ -203,12 +212,32 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
                 let url_string = input.expect_url_or_string()?.as_ref().to_owned();
                 let url = CssUrl::parse_from_string(url_string, &self.context, CorsMode::None);
 
+                #[cfg(feature = "gecko")]
+                let layers_enabled = static_prefs::pref!("layout.css.cascade-layers.enabled");
+                #[cfg(feature = "servo")]
+                let layers_enabled = false;
+
+                let layer = if !layers_enabled {
+                    None
+                } else if input.try_parse(|input| input.expect_ident_matching("layer")).is_ok() {
+                    Some(ImportLayer {
+                        name: None,
+                    })
+                } else {
+                    input.try_parse(|input| {
+                        input.expect_function_matching("layer")?;
+                        input.parse_nested_block(|input| {
+                            LayerName::parse(&self.context, input)
+                        }).map(|name| ImportLayer {
+                            name: Some(name),
+                        })
+                    }).ok()
+                };
+
                 let media = MediaList::parse(&self.context, input);
                 let media = Arc::new(self.shared_lock.wrap(media));
 
-                let prelude = AtRulePrelude::Import(url, media);
-
-                return Ok(prelude);
+                return Ok(AtRulePrelude::Import(url, media, layer));
             },
             "namespace" => {
                 if !self.check_state(State::Namespaces) {
@@ -225,8 +254,7 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
                     Err(e) => return Err(e.into()),
                 };
                 let url = Namespace::from(maybe_namespace.as_ref());
-                let prelude = AtRulePrelude::Namespace(prefix, url);
-                return Ok(prelude);
+                return Ok(AtRulePrelude::Namespace(prefix, url));
             },
             // @charset is removed by rust-cssparser if it’s the first rule in the stylesheet
             // anything left is invalid.
@@ -261,9 +289,9 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
         &mut self,
         prelude: AtRulePrelude,
         start: &ParserState,
-    ) -> Result<Self::AtRule, ()> {
+    ) -> Result<Self::AtRule, ()>  {
         let rule = match prelude {
-            AtRulePrelude::Import(url, media) => {
+            AtRulePrelude::Import(url, media, layer) => {
                 let loader = self
                     .loader
                     .expect("Expected a stylesheet loader for @import");
@@ -274,6 +302,7 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
                     &self.context,
                     &self.shared_lock,
                     media,
+                    layer,
                 );
 
                 self.state = State::Imports;
@@ -295,7 +324,19 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
                     source_location: start.source_location(),
                 })))
             },
-            _ => return Err(()),
+            AtRulePrelude::Layer(ref names) => {
+                if names.is_empty() {
+                    return Err(());
+                }
+                if self.state <= State::EarlyLayers {
+                    self.state = State::EarlyLayers;
+                } else {
+                    self.state = State::Body;
+                }
+                AtRuleParser::rule_without_block(&mut self.nested(), prelude, start)
+                    .expect("All validity checks on the nested parser should be done before changing self.state")
+            },
+            _ => AtRuleParser::rule_without_block(&mut self.nested(), prelude, start)?,
         };
 
         Ok((start.position(), rule))
@@ -379,41 +420,37 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
         name: CowRcStr<'i>,
         input: &mut Parser<'i, 't>,
     ) -> Result<Self::Prelude, ParseError<'i>> {
-        match_ignore_ascii_case! { &*name,
+        Ok(match_ignore_ascii_case! { &*name,
             "media" => {
                 let media_queries = MediaList::parse(self.context, input);
                 let arc = Arc::new(self.shared_lock.wrap(media_queries));
-                Ok(Self::Prelude::Media(arc))
+                AtRulePrelude::Media(arc)
             },
             "supports" => {
                 let cond = SupportsCondition::parse(input)?;
-                Ok(Self::Prelude::Supports(cond))
+                AtRulePrelude::Supports(cond)
             },
             "font-face" => {
-                Ok(Self::Prelude::FontFace)
+                AtRulePrelude::FontFace
             },
-            "font-feature-values" => {
-                if !cfg!(feature = "gecko") {
-                    // Support for this rule is not fully implemented in Servo yet.
-                    return Err(input.new_custom_error(StyleParseErrorKind::UnsupportedAtRule(name.clone())))
-                }
+            "layer" => {
+                let names = input.try_parse(|input| {
+                    input.parse_comma_separated(|input| {
+                        LayerName::parse(self.context, input)
+                    })
+                }).unwrap_or_default();
+                AtRulePrelude::Layer(names)
+            },
+            "font-feature-values" if cfg!(feature = "gecko") => {
                 let family_names = parse_family_name_list(self.context, input)?;
-                Ok(Self::Prelude::FontFeatureValues(family_names))
+                AtRulePrelude::FontFeatureValues(family_names)
             },
-            "counter-style" => {
-                if !cfg!(feature = "gecko") {
-                    // Support for this rule is not fully implemented in Servo yet.
-                    return Err(input.new_custom_error(StyleParseErrorKind::UnsupportedAtRule(name.clone())))
-                }
+            "counter-style" if cfg!(feature = "gecko") => {
                 let name = parse_counter_style_name_definition(input)?;
-                Ok(Self::Prelude::CounterStyle(name))
+                AtRulePrelude::CounterStyle(name)
             },
-            "viewport" => {
-                if viewport_rule::enabled() {
-                    Ok(Self::Prelude::Viewport)
-                } else {
-                    Err(input.new_custom_error(StyleParseErrorKind::UnsupportedAtRule(name.clone())))
-                }
+            "viewport" if viewport_rule::enabled() => {
+                AtRulePrelude::Viewport
             },
             "keyframes" | "-webkit-keyframes" | "-moz-keyframes" => {
                 let prefix = if starts_with_ignore_ascii_case(&*name, "-webkit-") {
@@ -429,28 +466,22 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
                     return Err(input.new_custom_error(StyleParseErrorKind::UnsupportedAtRule(name.clone())))
                 }
                 let name = KeyframesName::parse(self.context, input)?;
-
-                Ok(Self::Prelude::Keyframes(name, prefix))
+                AtRulePrelude::Keyframes(name, prefix)
             },
-            "page" => {
-                if cfg!(feature = "gecko") {
-                    Ok(Self::Prelude::Page)
-                } else {
-                    Err(input.new_custom_error(StyleParseErrorKind::UnsupportedAtRule(name.clone())))
-                }
+            "page" if cfg!(feature = "gecko") => {
+                AtRulePrelude::Page
             },
-            "-moz-document" => {
-                if !cfg!(feature = "gecko") {
-                    return Err(input.new_custom_error(
-                        StyleParseErrorKind::UnsupportedAtRule(name.clone())
-                    ))
-                }
-
+            "-moz-document" if cfg!(feature = "gecko") => {
                 let cond = DocumentCondition::parse(self.context, input)?;
-                Ok(Self::Prelude::Document(cond))
+                AtRulePrelude::Document(cond)
             },
-            _ => Err(input.new_custom_error(StyleParseErrorKind::UnsupportedAtRule(name.clone())))
-        }
+            #[cfg(feature = "gecko")]
+            "scroll-timeline" if static_prefs::pref!("layout.css.scroll-linked-animations.enabled") => {
+                let name = TimelineName::parse(self.context, input)?;
+                AtRulePrelude::ScrollTimeline(name)
+            },
+            _ => return Err(input.new_custom_error(StyleParseErrorKind::UnsupportedAtRule(name.clone())))
+        })
     }
 
     fn parse_block<'t>(
@@ -577,14 +608,62 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
                     },
                 ))))
             },
-            _ => Err(ParseError {
-                kind: ParseErrorKind::Basic(BasicParseErrorKind::AtRuleInvalid(CowRcStr::from(
-                    "Unsupported AtRule Prelude.",
-                ))),
-                location: start.source_location(),
-            }),
+            AtRulePrelude::Layer(names) => {
+                let name = match names.len() {
+                    0 | 1 => names.into_iter().next(),
+                    _ => return Err(input.new_error(BasicParseErrorKind::AtRuleBodyInvalid)),
+                };
+                Ok(CssRule::Layer(Arc::new(self.shared_lock.wrap(
+                    LayerRule {
+                        kind: LayerRuleKind::Block {
+                            name,
+                            rules: self.parse_nested_rules(input, CssRuleType::Layer),
+                        },
+                        source_location: start.source_location(),
+                    },
+                ))))
+            },
+            AtRulePrelude::Import(..) | AtRulePrelude::Namespace(..) => {
+                // These rules don't have blocks.
+                Err(input.new_unexpected_token_error(cssparser::Token::CurlyBracketBlock))
+            },
+            AtRulePrelude::ScrollTimeline(name) => {
+                let context = ParserContext::new_with_rule_type(
+                    self.context,
+                    CssRuleType::ScrollTimeline,
+                    self.namespaces,
+                );
+
+                Ok(CssRule::ScrollTimeline(Arc::new(self.shared_lock.wrap(
+                    ScrollTimelineRule {
+                        name,
+                        descriptors: ScrollTimelineDescriptors::parse(&context, input)?,
+                        source_location: start.source_location(),
+                    },
+                ))))
+            },
         }
     }
+
+    #[inline]
+    fn rule_without_block(
+        &mut self,
+        prelude: AtRulePrelude,
+        start: &ParserState,
+    ) -> Result<Self::AtRule, ()>  {
+        Ok(match prelude {
+            AtRulePrelude::Layer(names) => {
+                if names.is_empty() {
+                    return Err(());
+                }
+                CssRule::Layer(Arc::new(self.shared_lock.wrap(LayerRule {
+                    kind: LayerRuleKind::Statement { names },
+                    source_location: start.source_location(),
+                })))
+            },
+            _ => return Err(()),
+        })
+    }
 }
 
 #[inline(never)]
diff --git a/components/style/stylesheets/rules_iterator.rs b/components/style/stylesheets/rules_iterator.rs
index a7010ff066e..32851dd2cca 100644
--- a/components/style/stylesheets/rules_iterator.rs
+++ b/components/style/stylesheets/rules_iterator.rs
@@ -51,60 +51,68 @@ where
     pub fn skip_children(&mut self) {
         self.stack.pop();
     }
-}
 
-fn children_of_rule<'a, C>(
-    rule: &'a CssRule,
-    device: &'a Device,
-    quirks_mode: QuirksMode,
-    guard: &'a SharedRwLockReadGuard<'_>,
-    effective: &mut bool,
-) -> Option<slice::Iter<'a, CssRule>>
-where
-    C: NestedRuleIterationCondition + 'static,
-{
-    *effective = true;
-    match *rule {
-        CssRule::Namespace(_) |
-        CssRule::Style(_) |
-        CssRule::FontFace(_) |
-        CssRule::CounterStyle(_) |
-        CssRule::Viewport(_) |
-        CssRule::Keyframes(_) |
-        CssRule::Page(_) |
-        CssRule::FontFeatureValues(_) => None,
-        CssRule::Import(ref import_rule) => {
-            let import_rule = import_rule.read_with(guard);
-            if !C::process_import(guard, device, quirks_mode, import_rule) {
-                *effective = false;
-                return None;
+    /// Returns the children of `rule`, and whether `rule` is effective.
+    pub fn children(
+        rule: &'a CssRule,
+        device: &'a Device,
+        quirks_mode: QuirksMode,
+        guard: &'a SharedRwLockReadGuard<'_>,
+        effective: &mut bool,
+    ) -> Option<slice::Iter<'a, CssRule>> {
+        *effective = true;
+        match *rule {
+            CssRule::Namespace(_) |
+            CssRule::Style(_) |
+            CssRule::FontFace(_) |
+            CssRule::CounterStyle(_) |
+            CssRule::Viewport(_) |
+            CssRule::Keyframes(_) |
+            CssRule::ScrollTimeline(_) |
+            CssRule::Page(_) |
+            CssRule::FontFeatureValues(_) => None,
+            CssRule::Import(ref import_rule) => {
+                let import_rule = import_rule.read_with(guard);
+                if !C::process_import(guard, device, quirks_mode, import_rule) {
+                    *effective = false;
+                    return None;
+                }
+                Some(import_rule.stylesheet.rules(guard).iter())
+            },
+            CssRule::Document(ref doc_rule) => {
+                let doc_rule = doc_rule.read_with(guard);
+                if !C::process_document(guard, device, quirks_mode, doc_rule) {
+                    *effective = false;
+                    return None;
+                }
+                Some(doc_rule.rules.read_with(guard).0.iter())
+            },
+            CssRule::Media(ref lock) => {
+                let media_rule = lock.read_with(guard);
+                if !C::process_media(guard, device, quirks_mode, media_rule) {
+                    *effective = false;
+                    return None;
+                }
+                Some(media_rule.rules.read_with(guard).0.iter())
+            },
+            CssRule::Supports(ref lock) => {
+                let supports_rule = lock.read_with(guard);
+                if !C::process_supports(guard, device, quirks_mode, supports_rule) {
+                    *effective = false;
+                    return None;
+                }
+                Some(supports_rule.rules.read_with(guard).0.iter())
+            },
+            CssRule::Layer(ref lock) => {
+                use crate::stylesheets::layer_rule::LayerRuleKind;
+
+                let layer_rule = lock.read_with(guard);
+                match layer_rule.kind {
+                    LayerRuleKind::Block { ref rules, .. } => Some(rules.read_with(guard).0.iter()),
+                    LayerRuleKind::Statement { .. } => None,
+                }
             }
-            Some(import_rule.stylesheet.rules(guard).iter())
-        },
-        CssRule::Document(ref doc_rule) => {
-            let doc_rule = doc_rule.read_with(guard);
-            if !C::process_document(guard, device, quirks_mode, doc_rule) {
-                *effective = false;
-                return None;
-            }
-            Some(doc_rule.rules.read_with(guard).0.iter())
-        },
-        CssRule::Media(ref lock) => {
-            let media_rule = lock.read_with(guard);
-            if !C::process_media(guard, device, quirks_mode, media_rule) {
-                *effective = false;
-                return None;
-            }
-            Some(media_rule.rules.read_with(guard).0.iter())
-        },
-        CssRule::Supports(ref lock) => {
-            let supports_rule = lock.read_with(guard);
-            if !C::process_supports(guard, device, quirks_mode, supports_rule) {
-                *effective = false;
-                return None;
-            }
-            Some(supports_rule.rules.read_with(guard).0.iter())
-        },
+        }
     }
 }
 
@@ -129,7 +137,7 @@ where
             };
 
             let mut effective = true;
-            let children = children_of_rule::<C>(
+            let children = Self::children(
                 rule,
                 self.device,
                 self.quirks_mode,
@@ -315,7 +323,7 @@ impl<'a, 'b> EffectiveRulesIterator<'a, 'b> {
         guard: &'a SharedRwLockReadGuard<'b>,
         rule: &'a CssRule,
     ) -> Self {
-        let children = children_of_rule::<AllRules>(rule, device, quirks_mode, guard, &mut false);
+        let children = RulesIterator::<AllRules>::children(rule, device, quirks_mode, guard, &mut false);
         EffectiveRulesIterator::new(device, quirks_mode, guard, children.unwrap_or([].iter()))
     }
 }
diff --git a/components/style/stylesheets/scroll_timeline_rule.rs b/components/style/stylesheets/scroll_timeline_rule.rs
new file mode 100644
index 00000000000..bbc5d9caf8f
--- /dev/null
+++ b/components/style/stylesheets/scroll_timeline_rule.rs
@@ -0,0 +1,327 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! scroll-timeline-at-rule: https://drafts.csswg.org/scroll-animations/#scroll-timeline-at-rule
+
+use crate::parser::{Parse, ParserContext};
+use crate::shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
+use crate::str::CssStringWriter;
+use crate::values::specified::{LengthPercentage, Number};
+use crate::values::{AtomIdent, TimelineName};
+use cssparser::{AtRuleParser, CowRcStr, DeclarationParser, Parser, SourceLocation, Token};
+use selectors::parser::SelectorParseErrorKind;
+use std::fmt::{self, Debug, Write};
+use style_traits::{CssWriter, ParseError, StyleParseErrorKind, ToCss};
+
+/// A [`@scroll-timeline`][descriptors] rule.
+///
+/// [descriptors] https://drafts.csswg.org/scroll-animations/#scroll-timeline-descriptors
+#[derive(Clone, Debug, ToShmem)]
+pub struct ScrollTimelineRule {
+    /// The name of the current scroll timeline.
+    pub name: TimelineName,
+    /// The descriptors.
+    pub descriptors: ScrollTimelineDescriptors,
+    /// The line and column of the rule's source code.
+    pub source_location: SourceLocation,
+}
+
+impl ToCssWithGuard for ScrollTimelineRule {
+    fn to_css(&self, _guard: &SharedRwLockReadGuard, dest: &mut CssStringWriter) -> fmt::Result {
+        let mut dest = CssWriter::new(dest);
+        dest.write_str("@scroll-timeline ")?;
+        self.name.to_css(&mut dest)?;
+        dest.write_str(" { ")?;
+        self.descriptors.to_css(&mut dest)?;
+        dest.write_str("}")
+    }
+}
+
+/// The descriptors of @scroll-timeline.
+///
+/// https://drafts.csswg.org/scroll-animations/#scroll-timeline-descriptors
+#[derive(Clone, Debug, Default, ToShmem)]
+pub struct ScrollTimelineDescriptors {
+    /// The source of the current scroll timeline.
+    pub source: Option<Source>,
+    /// The orientation of the current scroll timeline.
+    pub orientation: Option<Orientation>,
+    /// The scroll timeline's scrollOffsets.
+    pub offsets: Option<ScrollOffsets>,
+}
+
+impl Parse for ScrollTimelineDescriptors {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        use crate::cssparser::DeclarationListParser;
+        use crate::error_reporting::ContextualParseError;
+
+        let mut descriptors = ScrollTimelineDescriptors::default();
+        let parser = ScrollTimelineDescriptorsParser {
+            context,
+            descriptors: &mut descriptors,
+        };
+        let mut iter = DeclarationListParser::new(input, parser);
+        while let Some(declaration) = iter.next() {
+            if let Err((error, slice)) = declaration {
+                let location = error.location;
+                let error = ContextualParseError::UnsupportedRule(slice, error);
+                context.log_css_error(location, error)
+            }
+        }
+
+        Ok(descriptors)
+    }
+}
+
+// Basically, this is used for the serialization of CSSScrollTimelineRule, so we follow the
+// instructions in https://drafts.csswg.org/scroll-animations-1/#serialize-a-cssscrolltimelinerule.
+impl ToCss for ScrollTimelineDescriptors {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        if let Some(ref value) = self.source {
+            dest.write_str("source: ")?;
+            value.to_css(dest)?;
+            dest.write_str("; ")?;
+        }
+
+        if let Some(ref value) = self.orientation {
+            dest.write_str("orientation: ")?;
+            value.to_css(dest)?;
+            dest.write_str("; ")?;
+        }
+
+        // https://github.com/w3c/csswg-drafts/issues/6617
+        if let Some(ref value) = self.offsets {
+            dest.write_str("scroll-offsets: ")?;
+            value.to_css(dest)?;
+            dest.write_str("; ")?;
+        }
+        Ok(())
+    }
+}
+
+struct ScrollTimelineDescriptorsParser<'a, 'b: 'a> {
+    context: &'a ParserContext<'b>,
+    descriptors: &'a mut ScrollTimelineDescriptors,
+}
+
+impl<'a, 'b, 'i> AtRuleParser<'i> for ScrollTimelineDescriptorsParser<'a, 'b> {
+    type Prelude = ();
+    type AtRule = ();
+    type Error = StyleParseErrorKind<'i>;
+}
+
+impl<'a, 'b, 'i> DeclarationParser<'i> for ScrollTimelineDescriptorsParser<'a, 'b> {
+    type Declaration = ();
+    type Error = StyleParseErrorKind<'i>;
+
+    fn parse_value<'t>(
+        &mut self,
+        name: CowRcStr<'i>,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<(), ParseError<'i>> {
+        macro_rules! parse_descriptor {
+            (
+                $( $name: tt / $ident: ident, )*
+            ) => {
+                match_ignore_ascii_case! { &*name,
+                    $(
+                        $name => {
+                            let value = input.parse_entirely(|i| Parse::parse(self.context, i))?;
+                            self.descriptors.$ident = Some(value)
+                        },
+                    )*
+                    _ => {
+                        return Err(input.new_custom_error(
+                            SelectorParseErrorKind::UnexpectedIdent(name.clone()),
+                        ))
+                    }
+                }
+            }
+        }
+        parse_descriptor! {
+            "source" / source,
+            "orientation" / orientation,
+            "scroll-offsets" / offsets,
+        };
+        Ok(())
+    }
+}
+
+/// The scroll-timeline source.
+///
+/// https://drafts.csswg.org/scroll-animations/#descdef-scroll-timeline-source
+#[derive(Clone, Debug, Parse, PartialEq, ToCss, ToShmem)]
+pub enum Source {
+    /// The scroll container.
+    Selector(ScrollTimelineSelector),
+    /// The initial value. The scrollingElement of the Document associated with the Window that is
+    /// the current global object.
+    Auto,
+    /// Null. However, it's not clear what is the expected behavior of this. See the spec issue:
+    /// https://drafts.csswg.org/scroll-animations/#issue-0d1e73bd
+    None,
+}
+
+impl Default for Source {
+    fn default() -> Self {
+        Source::Auto
+    }
+}
+
+/// The scroll-timeline orientation.
+/// https://drafts.csswg.org/scroll-animations/#descdef-scroll-timeline-orientation
+///
+/// Note: the initial orientation is auto, and we will treat it as block, the same as the
+/// definition of ScrollTimelineOptions (WebIDL API).
+/// https://drafts.csswg.org/scroll-animations/#dom-scrolltimelineoptions-orientation
+#[derive(Clone, Copy, Debug, MallocSizeOf, Eq, Parse, PartialEq, PartialOrd, ToCss, ToShmem)]
+pub enum Orientation {
+    /// The initial value.
+    Auto,
+    /// The direction along the block axis. This is the default value.
+    Block,
+    /// The direction along the inline axis
+    Inline,
+    /// The physical horizontal direction.
+    Horizontal,
+    /// The physical vertical direction.
+    Vertical,
+}
+
+impl Default for Orientation {
+    fn default() -> Self {
+        Orientation::Auto
+    }
+}
+
+/// Scroll-timeline offsets. We treat None as an empty vector.
+/// value: none | <scroll-timeline-offset>#
+///
+/// https://drafts.csswg.org/scroll-animations/#descdef-scroll-timeline-scroll-offsets
+#[derive(Clone, Default, Debug, ToCss, ToShmem)]
+#[css(comma)]
+pub struct ScrollOffsets(#[css(if_empty = "none", iterable)] Box<[ScrollTimelineOffset]>);
+
+impl Parse for ScrollOffsets {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        if input.try_parse(|i| i.expect_ident_matching("none")).is_ok() {
+            return Ok(ScrollOffsets(Box::new([])));
+        }
+
+        Ok(ScrollOffsets(
+            input
+                .parse_comma_separated(|i| ScrollTimelineOffset::parse(context, i))?
+                .into_boxed_slice(),
+        ))
+    }
+}
+
+/// A <scroll-timeline-offset>.
+/// value: auto | <length-percentage> | <element-offset>
+///
+/// https://drafts.csswg.org/scroll-animations/#typedef-scroll-timeline-offset
+#[derive(Clone, Debug, Parse, PartialEq, ToCss, ToShmem)]
+pub enum ScrollTimelineOffset {
+    /// The initial value. A container-based offset.
+    Auto,
+    /// A container-based offset with the distance indicated by the value along source's scroll
+    /// range in orientation.
+    LengthPercentage(LengthPercentage),
+    /// An element-based offset.
+    ElementOffset(ElementOffset),
+}
+
+/// An <element-offset-edge>.
+///
+/// https://drafts.csswg.org/scroll-animations-1/#typedef-element-offset-edge
+#[derive(Clone, Copy, Debug, MallocSizeOf, Eq, Parse, PartialEq, PartialOrd, ToCss, ToShmem)]
+pub enum ElementOffsetEdge {
+    /// Start edge
+    Start,
+    /// End edge.
+    End,
+}
+
+/// An <element-offset>.
+/// value: selector( <id-selector> ) [<element-offset-edge> || <number>]?
+///
+/// https://drafts.csswg.org/scroll-animations-1/#typedef-element-offset
+#[derive(Clone, Debug, PartialEq, ToCss, ToShmem)]
+pub struct ElementOffset {
+    /// The target whose intersection with source's scrolling box determines the concrete scroll
+    /// offset.
+    target: ScrollTimelineSelector,
+    /// An optional value of <element-offset-edge>. If not provided, the default value is start.
+    edge: Option<ElementOffsetEdge>,
+    /// An optional value of threshold. If not provided, the default value is 0.
+    threshold: Option<Number>,
+}
+
+impl Parse for ElementOffset {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        let target = ScrollTimelineSelector::parse(context, input)?;
+
+        // Parse `[<element-offset-edge> || <number>]?`
+        let mut edge = input.try_parse(ElementOffsetEdge::parse).ok();
+        let threshold = input.try_parse(|i| Number::parse(context, i)).ok();
+        if edge.is_none() {
+            edge = input.try_parse(ElementOffsetEdge::parse).ok();
+        }
+
+        Ok(ElementOffset {
+            target,
+            edge,
+            threshold,
+        })
+    }
+}
+
+/// The type of the selector ID.
+#[derive(Clone, Eq, PartialEq, ToShmem)]
+pub struct ScrollTimelineSelector(AtomIdent);
+
+impl Parse for ScrollTimelineSelector {
+    fn parse<'i, 't>(
+        _context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        // Parse `selector(<id-selector>)`.
+        input.expect_function_matching("selector")?;
+        input.parse_nested_block(|i| match i.next()? {
+            Token::IDHash(id) => Ok(ScrollTimelineSelector(id.as_ref().into())),
+            _ => Err(i.new_custom_error(StyleParseErrorKind::UnspecifiedError)),
+        })
+    }
+}
+
+impl ToCss for ScrollTimelineSelector {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        use crate::cssparser::ToCss as CssparserToCss;
+        dest.write_str("selector(")?;
+        dest.write_char('#')?;
+        self.0.to_css(dest)?;
+        dest.write_char(')')
+    }
+}
+
+impl Debug for ScrollTimelineSelector {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.to_css(&mut CssWriter::new(f))
+    }
+}
diff --git a/components/style/stylesheets/stylesheet.rs b/components/style/stylesheets/stylesheet.rs
index b8e7f246c19..1b6b59bac78 100644
--- a/components/style/stylesheets/stylesheet.rs
+++ b/components/style/stylesheets/stylesheet.rs
@@ -278,15 +278,8 @@ pub trait StylesheetInDocument: ::std::fmt::Debug {
 
     rule_filter! {
         effective_style_rules(Style => StyleRule),
-        effective_media_rules(Media => MediaRule),
         effective_font_face_rules(FontFace => FontFaceRule),
-        effective_font_face_feature_values_rules(FontFeatureValues => FontFeatureValuesRule),
-        effective_counter_style_rules(CounterStyle => CounterStyleRule),
         effective_viewport_rules(Viewport => ViewportRule),
-        effective_keyframes_rules(Keyframes => KeyframesRule),
-        effective_supports_rules(Supports => SupportsRule),
-        effective_page_rules(Page => PageRule),
-        effective_document_rules(Document => DocumentRule),
     }
 }
 
@@ -367,7 +360,10 @@ impl SanitizationKind {
             CssRule::Document(..) |
             CssRule::Media(..) |
             CssRule::Supports(..) |
-            CssRule::Import(..) => false,
+            CssRule::Import(..) |
+            // TODO(emilio): Perhaps Layer should not be always sanitized? But
+            // we sanitize @media and co, so this seems safer for now.
+            CssRule::Layer(..) => false,
 
             CssRule::FontFace(..) | CssRule::Namespace(..) | CssRule::Style(..) => true,
 
@@ -375,7 +371,8 @@ impl SanitizationKind {
             CssRule::Page(..) |
             CssRule::FontFeatureValues(..) |
             CssRule::Viewport(..) |
-            CssRule::CounterStyle(..) => !is_standard,
+            CssRule::CounterStyle(..) |
+            CssRule::ScrollTimeline(..) => !is_standard,
         }
     }
 }
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index fb7334d3600..0663ea440d9 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -25,11 +25,12 @@ use crate::shared_lock::{Locked, SharedRwLockReadGuard, StylesheetGuards};
 use crate::stylesheet_set::{DataValidity, DocumentStylesheetSet, SheetRebuildKind};
 use crate::stylesheet_set::{DocumentStylesheetFlusher, SheetCollectionFlusher};
 use crate::stylesheets::keyframes_rule::KeyframesAnimation;
+use crate::stylesheets::layer_rule::{LayerName, LayerId, LayerOrder};
 use crate::stylesheets::viewport_rule::{self, MaybeNew, ViewportRule};
 use crate::stylesheets::{StyleRule, StylesheetInDocument, StylesheetContents};
 #[cfg(feature = "gecko")]
 use crate::stylesheets::{CounterStyleRule, FontFaceRule, FontFeatureValuesRule, PageRule};
-use crate::stylesheets::{CssRule, Origin, OriginSet, PerOrigin, PerOriginIter};
+use crate::stylesheets::{CssRule, Origin, OriginSet, PerOrigin, PerOriginIter, EffectiveRulesIterator};
 use crate::thread_state::{self, ThreadState};
 use crate::{Atom, LocalName, Namespace, WeakAtom};
 use fallible::FallibleVec;
@@ -292,6 +293,8 @@ impl CascadeDataCacheEntry for UserAgentCascadeData {
             )?;
         }
 
+        new_data.cascade_data.compute_layer_order();
+
         Ok(Arc::new(new_data))
     }
 
@@ -1866,6 +1869,23 @@ impl PartElementAndPseudoRules {
     }
 }
 
+#[derive(Clone, Debug, MallocSizeOf)]
+struct CascadeLayer {
+    id: LayerId,
+    order: LayerOrder,
+    children: Vec<LayerId>,
+}
+
+impl CascadeLayer {
+    const fn root() -> Self {
+        Self {
+            id: LayerId::root(),
+            order: LayerOrder::root(),
+            children: vec![],
+        }
+    }
+}
+
 /// Data resulting from performing the CSS cascade that is specific to a given
 /// origin.
 ///
@@ -1931,6 +1951,12 @@ pub struct CascadeData {
     /// by name.
     animations: PrecomputedHashMap<Atom, KeyframesAnimation>,
 
+    /// A map from cascade layer name to layer order.
+    layer_id: FxHashMap<LayerName, LayerId>,
+
+    /// The list of cascade layers, indexed by their layer id.
+    layers: SmallVec<[CascadeLayer; 1]>,
+
     /// Effective media query results cached from the last rebuild.
     effective_media_query_results: EffectiveMediaQueryResults,
 
@@ -1961,8 +1987,18 @@ impl CascadeData {
             state_dependencies: ElementState::empty(),
             document_state_dependencies: DocumentState::empty(),
             mapped_ids: PrecomputedHashSet::default(),
-            selectors_for_cache_revalidation: SelectorMap::new(),
+            // NOTE: We disable attribute bucketing for revalidation because we
+            // rely on the buckets to match, but we don't want to just not share
+            // style across elements with different attributes.
+            //
+            // An alternative to this would be to perform a style sharing check
+            // like may_match_different_id_rules which would check that the
+            // attribute buckets match on all scopes. But that seems
+            // somewhat gnarly.
+            selectors_for_cache_revalidation: SelectorMap::new_without_attribute_bucketing(),
             animations: Default::default(),
+            layer_id: Default::default(),
+            layers: smallvec::smallvec![CascadeLayer::root()],
             extra_data: ExtraStyleData::default(),
             effective_media_query_results: EffectiveMediaQueryResults::new(),
             rules_source_order: 0,
@@ -2009,6 +2045,8 @@ impl CascadeData {
             result.is_ok()
         });
 
+        self.compute_layer_order();
+
         result
     }
 
@@ -2070,6 +2108,43 @@ impl CascadeData {
         self.part_rules.is_some()
     }
 
+    #[inline]
+    fn layer_order_for(&self, id: LayerId) -> LayerOrder {
+        self.layers[id.0 as usize].order
+    }
+
+    fn compute_layer_order(&mut self) {
+        debug_assert_ne!(self.layers.len(), 0, "There should be at least the root layer!");
+        if self.layers.len() == 1 {
+            return; // Nothing to do
+        }
+        let (first, remaining) = self.layers.split_at_mut(1);
+        let root = &mut first[0];
+        let mut order = LayerOrder::first();
+        compute_layer_order_for_subtree(root, remaining, &mut order);
+
+        // NOTE(emilio): This is a bit trickier than it should to avoid having
+        // to clone() around layer indices.
+        fn compute_layer_order_for_subtree(
+            parent: &mut CascadeLayer,
+            remaining_layers: &mut [CascadeLayer],
+            order: &mut LayerOrder,
+        ) {
+            for child in parent.children.iter() {
+                debug_assert!(parent.id < *child, "Children are always registered after parents");
+                let child_index = (child.0 - parent.id.0 - 1) as usize;
+                let (first, remaining) = remaining_layers.split_at_mut(child_index + 1);
+                let child = &mut first[child_index];
+                compute_layer_order_for_subtree(child, remaining, order);
+            }
+
+            if parent.id != LayerId::root() {
+                parent.order = *order;
+                order.inc();
+            }
+        }
+    }
+
     /// Collects all the applicable media query results into `results`.
     ///
     /// This duplicates part of the logic in `add_stylesheet`, which is
@@ -2117,31 +2192,25 @@ impl CascadeData {
         }
     }
 
-    // Returns Err(..) to signify OOM
-    fn add_stylesheet<S>(
+    fn add_rule_list<S>(
         &mut self,
+        rules: std::slice::Iter<'_, CssRule>,
         device: &Device,
         quirks_mode: QuirksMode,
         stylesheet: &S,
         guard: &SharedRwLockReadGuard,
         rebuild_kind: SheetRebuildKind,
+        mut current_layer: &mut LayerName,
+        current_layer_id: LayerId,
         mut precomputed_pseudo_element_decls: Option<&mut PrecomputedPseudoElementDeclarations>,
     ) -> Result<(), FailedAllocationError>
     where
         S: StylesheetInDocument + 'static,
     {
-        if !stylesheet.enabled() || !stylesheet.is_effective_for_device(device, guard) {
-            return Ok(());
-        }
-
-        let contents = stylesheet.contents();
-        let origin = contents.origin;
-
-        if rebuild_kind.should_rebuild_invalidation() {
-            self.effective_media_query_results.saw_effective(contents);
-        }
-
-        for rule in stylesheet.effective_rules(device, guard) {
+        for rule in rules {
+            // Handle leaf rules first, as those are by far the most common
+            // ones, and are always effective, so we can skip some checks.
+            let mut handled = true;
             match *rule {
                 CssRule::Style(ref locked) => {
                     let style_rule = locked.read_with(&guard);
@@ -2154,7 +2223,8 @@ impl CascadeData {
                         if let Some(pseudo) = pseudo_element {
                             if pseudo.is_precomputed() {
                                 debug_assert!(selector.is_universal());
-                                debug_assert!(matches!(origin, Origin::UserAgent));
+                                debug_assert_eq!(stylesheet.contents().origin, Origin::UserAgent);
+                                debug_assert_eq!(current_layer_id, LayerId::root());
 
                                 precomputed_pseudo_element_decls
                                     .as_mut()
@@ -2165,6 +2235,7 @@ impl CascadeData {
                                         self.rules_source_order,
                                         CascadeLevel::UANormal,
                                         selector.specificity(),
+                                        LayerOrder::root(),
                                     ));
                                 continue;
                             }
@@ -2180,6 +2251,7 @@ impl CascadeData {
                             hashes,
                             locked.clone(),
                             self.rules_source_order,
+                            current_layer_id,
                         );
 
                         if rebuild_kind.should_rebuild_invalidation() {
@@ -2243,43 +2315,52 @@ impl CascadeData {
                     }
                     self.rules_source_order += 1;
                 },
-                CssRule::Import(ref lock) => {
-                    if rebuild_kind.should_rebuild_invalidation() {
-                        let import_rule = lock.read_with(guard);
-                        self.effective_media_query_results
-                            .saw_effective(import_rule);
-                    }
-
-                    // NOTE: effective_rules visits the inner stylesheet if
-                    // appropriate.
-                },
-                CssRule::Media(ref lock) => {
-                    if rebuild_kind.should_rebuild_invalidation() {
-                        let media_rule = lock.read_with(guard);
-                        self.effective_media_query_results.saw_effective(media_rule);
-                    }
-                },
                 CssRule::Keyframes(ref keyframes_rule) => {
+                    #[cfg(feature = "gecko")]
+                    use hashglobe::hash_map::Entry;
+                    #[cfg(feature = "servo")]
+                    use hashglobe::fake::Entry;
+
                     let keyframes_rule = keyframes_rule.read_with(guard);
                     debug!("Found valid keyframes rule: {:?}", *keyframes_rule);
-
-                    // Don't let a prefixed keyframes animation override a non-prefixed one.
-                    let needs_insertion = keyframes_rule.vendor_prefix.is_none() ||
-                        self.animations
-                            .get(keyframes_rule.name.as_atom())
-                            .map_or(true, |rule| rule.vendor_prefix.is_some());
-                    if needs_insertion {
-                        let animation = KeyframesAnimation::from_keyframes(
-                            &keyframes_rule.keyframes,
-                            keyframes_rule.vendor_prefix.clone(),
-                            guard,
-                        );
-                        debug!("Found valid keyframe animation: {:?}", animation);
-                        self.animations
-                            .try_insert(keyframes_rule.name.as_atom().clone(), animation)?;
+                    match self.animations.try_entry(keyframes_rule.name.as_atom().clone())? {
+                        Entry::Vacant(e) => {
+                            e.insert(KeyframesAnimation::from_keyframes(
+                                &keyframes_rule.keyframes,
+                                keyframes_rule.vendor_prefix.clone(),
+                                current_layer_id,
+                                guard,
+                            ));
+                        },
+                        Entry::Occupied(mut e) => {
+                            // Don't let a prefixed keyframes animation override
+                            // a non-prefixed one.
+                            //
+                            // TODO(emilio): This will need to be harder for
+                            // layers.
+                            let needs_insert =
+                                 keyframes_rule.vendor_prefix.is_none() ||
+                                 e.get().vendor_prefix.is_some();
+                            if needs_insert {
+                                e.insert(KeyframesAnimation::from_keyframes(
+                                    &keyframes_rule.keyframes,
+                                    keyframes_rule.vendor_prefix.clone(),
+                                    current_layer_id,
+                                    guard,
+                                ));
+                            }
+                        },
                     }
                 },
                 #[cfg(feature = "gecko")]
+                CssRule::ScrollTimeline(..) => {
+                    // TODO: Bug 1676791: set the timeline into animation.
+                    // https://phabricator.services.mozilla.com/D126452
+                    //
+                    // Note: Bug 1733260: we may drop @scroll-timeline rule once this spec issue
+                    // https://github.com/w3c/csswg-drafts/issues/6674 gets landed.
+                },
+                #[cfg(feature = "gecko")]
                 CssRule::FontFace(ref rule) => {
                     self.extra_data.add_font_face(rule);
                 },
@@ -2295,14 +2376,223 @@ impl CascadeData {
                 CssRule::Page(ref rule) => {
                     self.extra_data.add_page(rule);
                 },
+                CssRule::Viewport(..) => {},
+                _ => {
+                    handled = false;
+                },
+            }
+
+            if handled {
+                // Assert that there are no children, and that the rule is
+                // effective.
+                if cfg!(debug_assertions) {
+                    let mut effective = false;
+                    let children = EffectiveRulesIterator::children(
+                        rule,
+                        device,
+                        quirks_mode,
+                        guard,
+                        &mut effective,
+                    );
+                    debug_assert!(children.is_none());
+                    debug_assert!(effective);
+                }
+                continue;
+            }
+
+            let mut effective = false;
+            let children = EffectiveRulesIterator::children(
+                rule,
+                device,
+                quirks_mode,
+                guard,
+                &mut effective,
+            );
+
+            if !effective {
+                continue;
+            }
+
+            fn maybe_register_layer(data: &mut CascadeData, layer: &LayerName) -> LayerId {
+                // TODO: Measure what's more common / expensive, if
+                // layer.clone() or the double hash lookup in the insert
+                // case.
+                if let Some(id) = data.layer_id.get(layer) {
+                    return *id;
+                }
+                let id = LayerId(data.layers.len() as u32);
+
+                let parent_layer_id = if layer.layer_names().len() > 1 {
+                    let mut parent = layer.clone();
+                    parent.0.pop();
+
+                    *data.layer_id
+                        .get_mut(&parent)
+                        .expect("Parent layers should be registered before child layers")
+                } else {
+                    LayerId::root()
+                };
+
+                data.layers[parent_layer_id.0 as usize].children.push(id);
+                data.layers.push(CascadeLayer {
+                    id,
+                    // NOTE(emilio): Order is evaluated after rebuild in
+                    // compute_layer_order.
+                    order: LayerOrder::first(),
+                    children: vec![],
+                });
+
+                data.layer_id.insert(layer.clone(), id);
+
+                id
+            }
+
+            fn maybe_register_layers(
+                data: &mut CascadeData,
+                name: Option<&LayerName>,
+                current_layer: &mut LayerName,
+                pushed_layers: &mut usize,
+            ) -> LayerId {
+                let anon_name;
+                let name = match name {
+                    Some(name) => name,
+                    None => {
+                        anon_name = LayerName::new_anonymous();
+                        &anon_name
+                    },
+                };
+
+                let mut id = LayerId::root();
+                for name in name.layer_names() {
+                    current_layer.0.push(name.clone());
+                    id = maybe_register_layer(data, &current_layer);
+                    *pushed_layers += 1;
+                }
+                debug_assert_ne!(id, LayerId::root());
+                id
+            }
+
+            let mut layer_names_to_pop = 0;
+            let mut children_layer_id = current_layer_id;
+            match *rule {
+                CssRule::Import(ref lock) => {
+                    let import_rule = lock.read_with(guard);
+                    if rebuild_kind.should_rebuild_invalidation() {
+                        self.effective_media_query_results
+                            .saw_effective(import_rule);
+                    }
+                    if let Some(ref layer) = import_rule.layer {
+                        children_layer_id = maybe_register_layers(
+                            self,
+                            layer.name.as_ref(),
+                            &mut current_layer,
+                            &mut layer_names_to_pop,
+                        );
+                    }
+
+                },
+                CssRule::Media(ref lock) => {
+                    if rebuild_kind.should_rebuild_invalidation() {
+                        let media_rule = lock.read_with(guard);
+                        self.effective_media_query_results.saw_effective(media_rule);
+                    }
+                },
+                CssRule::Layer(ref lock) => {
+                    use crate::stylesheets::layer_rule::LayerRuleKind;
+
+                    let layer_rule = lock.read_with(guard);
+                    match layer_rule.kind {
+                        LayerRuleKind::Block { ref name, .. } => {
+                            children_layer_id = maybe_register_layers(
+                                self,
+                                name.as_ref(),
+                                &mut current_layer,
+                                &mut layer_names_to_pop,
+                            );
+                        }
+                        LayerRuleKind::Statement { ref names } => {
+                            for name in &**names {
+                                let mut pushed = 0;
+                                // There are no children, so we can ignore the
+                                // return value.
+                                maybe_register_layers(
+                                    self,
+                                    Some(name),
+                                    &mut current_layer,
+                                    &mut pushed,
+                                );
+                                for _ in 0..pushed {
+                                    current_layer.0.pop();
+                                }
+                            }
+                        }
+                    }
+                },
                 // We don't care about any other rule.
                 _ => {},
             }
+
+            if let Some(children) = children {
+                self.add_rule_list(
+                    children,
+                    device,
+                    quirks_mode,
+                    stylesheet,
+                    guard,
+                    rebuild_kind,
+                    current_layer,
+                    children_layer_id,
+                    precomputed_pseudo_element_decls.as_deref_mut(),
+                )?;
+            }
+
+            for _ in 0..layer_names_to_pop {
+                current_layer.0.pop();
+            }
         }
 
         Ok(())
     }
 
+    // Returns Err(..) to signify OOM
+    fn add_stylesheet<S>(
+        &mut self,
+        device: &Device,
+        quirks_mode: QuirksMode,
+        stylesheet: &S,
+        guard: &SharedRwLockReadGuard,
+        rebuild_kind: SheetRebuildKind,
+        mut precomputed_pseudo_element_decls: Option<&mut PrecomputedPseudoElementDeclarations>,
+    ) -> Result<(), FailedAllocationError>
+    where
+        S: StylesheetInDocument + 'static,
+    {
+        if !stylesheet.enabled() || !stylesheet.is_effective_for_device(device, guard) {
+            return Ok(());
+        }
+
+        let contents = stylesheet.contents();
+
+        if rebuild_kind.should_rebuild_invalidation() {
+            self.effective_media_query_results.saw_effective(contents);
+        }
+
+        let mut current_layer = LayerName::new_empty();
+        self.add_rule_list(
+            contents.rules(guard).iter(),
+            device,
+            quirks_mode,
+            stylesheet,
+            guard,
+            rebuild_kind,
+            &mut current_layer,
+            LayerId::root(),
+            precomputed_pseudo_element_decls.as_deref_mut(),
+        )?;
+
+        Ok(())
+    }
+
     /// Returns whether all the media-feature affected values matched before and
     /// match now in the given stylesheet.
     pub fn media_feature_affected_matches<S>(
@@ -2345,9 +2635,11 @@ impl CascadeData {
                 CssRule::CounterStyle(..) |
                 CssRule::Supports(..) |
                 CssRule::Keyframes(..) |
+                CssRule::ScrollTimeline(..) |
                 CssRule::Page(..) |
                 CssRule::Viewport(..) |
                 CssRule::Document(..) |
+                CssRule::Layer(..) |
                 CssRule::FontFeatureValues(..) => {
                     // Not affected by device changes.
                     continue;
@@ -2413,6 +2705,9 @@ impl CascadeData {
             host_rules.clear();
         }
         self.animations.clear();
+        self.layer_id.clear();
+        self.layers.clear();
+        self.layers.push(CascadeLayer::root());
         self.extra_data.clear();
         self.rules_source_order = 0;
         self.num_selectors = 0;
@@ -2501,6 +2796,9 @@ pub struct Rule {
     /// we could repurpose that storage here if we needed to.
     pub source_order: u32,
 
+    /// The current layer id of this style rule.
+    pub layer_id: LayerId,
+
     /// The actual style rule.
     #[cfg_attr(
         feature = "gecko",
@@ -2527,9 +2825,16 @@ impl Rule {
     pub fn to_applicable_declaration_block(
         &self,
         level: CascadeLevel,
+        cascade_data: &CascadeData,
     ) -> ApplicableDeclarationBlock {
         let source = StyleSource::from_rule(self.style_rule.clone());
-        ApplicableDeclarationBlock::new(source, self.source_order, level, self.specificity())
+        ApplicableDeclarationBlock::new(
+            source,
+            self.source_order,
+            level,
+            self.specificity(),
+            cascade_data.layer_order_for(self.layer_id),
+        )
     }
 
     /// Creates a new Rule.
@@ -2538,12 +2843,14 @@ impl Rule {
         hashes: AncestorHashes,
         style_rule: Arc<Locked<StyleRule>>,
         source_order: u32,
+        layer_id: LayerId,
     ) -> Self {
         Rule {
-            selector: selector,
-            hashes: hashes,
-            style_rule: style_rule,
-            source_order: source_order,
+            selector,
+            hashes,
+            style_rule,
+            source_order,
+            layer_id,
         }
     }
 }
diff --git a/components/style/values/computed/box.rs b/components/style/values/computed/box.rs
index 22f5e306976..ce467343005 100644
--- a/components/style/values/computed/box.rs
+++ b/components/style/values/computed/box.rs
@@ -11,14 +11,12 @@ use crate::values::generics::box_::Perspective as GenericPerspective;
 use crate::values::generics::box_::VerticalAlign as GenericVerticalAlign;
 use crate::values::specified::box_ as specified;
 
-pub use crate::values::specified::box_::Clear as SpecifiedClear;
-pub use crate::values::specified::box_::{AnimationName, Appearance, BreakBetween, BreakWithin};
-pub use crate::values::specified::box_::{Contain, Display, Float as SpecifiedFloat, Overflow};
-pub use crate::values::specified::box_::{OverflowAnchor, OverflowClipBox, OverscrollBehavior};
 pub use crate::values::specified::box_::{
-    ScrollSnapAlign, ScrollSnapAxis, ScrollSnapStrictness, ScrollSnapType,
+    AnimationName, AnimationTimeline, Appearance, BreakBetween, BreakWithin,
+    Clear as SpecifiedClear, Contain, Display, Float as SpecifiedFloat, Overflow, OverflowAnchor,
+    OverflowClipBox, OverscrollBehavior, ScrollSnapAlign, ScrollSnapAxis, ScrollSnapStrictness,
+    ScrollSnapType, TouchAction, TransitionProperty, WillChange,
 };
-pub use crate::values::specified::box_::{TouchAction, TransitionProperty, WillChange};
 
 /// A computed value for the `vertical-align` property.
 pub type VerticalAlign = GenericVerticalAlign<LengthPercentage>;
diff --git a/components/style/values/computed/font.rs b/components/style/values/computed/font.rs
index e2b29f61d5e..18fbe8c6fd8 100644
--- a/components/style/values/computed/font.rs
+++ b/components/style/values/computed/font.rs
@@ -439,6 +439,26 @@ pub enum GenericFontFamily {
     MozEmoji,
 }
 
+impl GenericFontFamily {
+    /// When we disallow websites to override fonts, we ignore some generic
+    /// families that the website might specify, since they're not configured by
+    /// the user. See bug 789788 and bug 1730098.
+    #[cfg(feature = "gecko")]
+    pub (crate) fn valid_for_user_font_prioritization(self) -> bool {
+        match self {
+            Self::None |
+            Self::Fantasy |
+            Self::Cursive |
+            Self::SystemUi |
+            Self::MozEmoji => false,
+
+            Self::Serif |
+            Self::SansSerif |
+            Self::Monospace => true,
+        }
+    }
+}
+
 impl Parse for SingleFontFamily {
     /// Parse a font-family value.
     fn parse<'i, 't>(context: &ParserContext, input: &mut Parser<'i, 't>) -> Result<Self, ParseError<'i>> {
@@ -579,14 +599,14 @@ impl FontFamilyList {
         self.list = crate::ArcSlice::from_iter(new_list.into_iter());
     }
 
-    /// If there's a generic font family on the list (which isn't cursive or
-    /// fantasy), then move it to the front of the list. Otherwise, prepend the
-    /// default generic.
+    /// If there's a generic font family on the list which is suitable for user
+    /// font prioritization, then move it to the front of the list. Otherwise,
+    /// prepend the default generic.
     #[cfg(feature = "gecko")]
     pub (crate) fn prioritize_first_generic_or_prepend(&mut self, generic: GenericFontFamily) {
         let index_of_first_generic = self.iter().position(|f| {
             match *f {
-                SingleFontFamily::Generic(f) => f != GenericFontFamily::Cursive && f != GenericFontFamily::Fantasy,
+                SingleFontFamily::Generic(f) => f.valid_for_user_font_prioritization(),
                 _ => false,
             }
         });
diff --git a/components/style/values/computed/image.rs b/components/style/values/computed/image.rs
index 6290ac68a5d..a0cb8f3fe45 100644
--- a/components/style/values/computed/image.rs
+++ b/components/style/values/computed/image.rs
@@ -24,6 +24,8 @@ use std::f32::consts::PI;
 use std::fmt::{self, Write};
 use style_traits::{CssWriter, ToCss};
 
+pub use specified::ImageRendering;
+
 /// Computed values for an image according to CSS-IMAGES.
 /// <https://drafts.csswg.org/css-images/#image-values>
 pub type Image =
diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index 3c4f7a2dbdc..02b42b48241 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -189,7 +189,7 @@ impl Size {
             #[cfg(feature = "gecko")]
             GenericSize::MinContent |
             GenericSize::MaxContent |
-            GenericSize::MozFitContent |
+            GenericSize::FitContent |
             GenericSize::MozAvailable |
             GenericSize::FitContentFunction(_) => false
         }
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index c87142ac1b5..955a1b93877 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -44,7 +44,7 @@ pub use self::basic_shape::FillRule;
 pub use self::border::{BorderCornerRadius, BorderRadius, BorderSpacing};
 pub use self::border::{BorderImageRepeat, BorderImageSideWidth};
 pub use self::border::{BorderImageSlice, BorderImageWidth};
-pub use self::box_::{AnimationIterationCount, AnimationName, Contain};
+pub use self::box_::{AnimationIterationCount, AnimationName, AnimationTimeline, Contain};
 pub use self::box_::{Appearance, BreakBetween, BreakWithin, Clear, Float};
 pub use self::box_::{Display, Overflow, OverflowAnchor, TransitionProperty};
 pub use self::box_::{OverflowClipBox, OverscrollBehavior, Perspective, Resize};
@@ -62,7 +62,7 @@ pub use self::font::{FontSize, FontSizeAdjust, FontStretch, FontSynthesis};
 pub use self::font::{FontVariantAlternates, FontWeight};
 pub use self::font::{FontVariantEastAsian, FontVariationSettings};
 pub use self::font::{MathDepth, MozScriptMinSize, MozScriptSizeMultiplier, XLang, XTextZoom};
-pub use self::image::{Gradient, Image, LineDirection, MozImageRect};
+pub use self::image::{Gradient, Image, LineDirection, MozImageRect, ImageRendering};
 pub use self::length::{CSSPixelLength, NonNegativeLength};
 pub use self::length::{Length, LengthOrNumber, LengthPercentage, NonNegativeLengthOrNumber};
 pub use self::length::{LengthOrAuto, LengthPercentageOrAuto, MaxSize, Size};
diff --git a/components/style/values/generics/length.rs b/components/style/values/generics/length.rs
index dfb3504e3cf..8d2596fffae 100644
--- a/components/style/values/generics/length.rs
+++ b/components/style/values/generics/length.rs
@@ -157,7 +157,7 @@ pub enum GenericSize<LengthPercent> {
     MinContent,
     #[cfg(feature = "gecko")]
     #[animation(error)]
-    MozFitContent,
+    FitContent,
     #[cfg(feature = "gecko")]
     #[animation(error)]
     MozAvailable,
@@ -207,15 +207,13 @@ pub enum GenericMaxSize<LengthPercent> {
     None,
     #[cfg(feature = "gecko")]
     #[animation(error)]
-    #[parse(aliases = "-moz-max-content")]
     MaxContent,
     #[cfg(feature = "gecko")]
     #[animation(error)]
-    #[parse(aliases = "-moz-min-content")]
     MinContent,
     #[cfg(feature = "gecko")]
     #[animation(error)]
-    MozFitContent,
+    FitContent,
     #[cfg(feature = "gecko")]
     #[animation(error)]
     MozAvailable,
diff --git a/components/style/values/generics/transform.rs b/components/style/values/generics/transform.rs
index 92987f89fe4..604872ba5dc 100644
--- a/components/style/values/generics/transform.rs
+++ b/components/style/values/generics/transform.rs
@@ -687,7 +687,7 @@ pub trait IsParallelTo {
 
 impl<Number, Angle> ToCss for Rotate<Number, Angle>
 where
-    Number: Copy + ToCss,
+    Number: Copy + ToCss + Zero,
     Angle: ToCss,
     (Number, Number, Number): IsParallelTo,
 {
@@ -700,25 +700,41 @@ where
             Rotate::None => dest.write_str("none"),
             Rotate::Rotate(ref angle) => angle.to_css(dest),
             Rotate::Rotate3D(x, y, z, ref angle) => {
-                // If a 3d rotation is specified, the property must serialize with an axis
-                // specified. If the axis is parallel with the x, y, or z axises, it must
-                // serialize as the appropriate keyword.
+                // If the axis is parallel with the x or y axes, it must serialize as the
+                // appropriate keyword. If a rotation about the z axis (that is, in 2D) is
+                // specified, the property must serialize as just an <angle>
+                //
                 // https://drafts.csswg.org/css-transforms-2/#individual-transform-serialization
                 let v = (x, y, z);
-                if v.is_parallel_to(&DirectionVector::new(1., 0., 0.)) {
-                    dest.write_char('x')?;
+                let axis = if x.is_zero() && y.is_zero() && z.is_zero() {
+                    // The zero length vector is parallel to every other vector, so
+                    // is_parallel_to() returns true for it. However, it is definitely different
+                    // from x axis, y axis, or z axis, and it's meaningless to perform a rotation
+                    // using that direction vector. So we *have* to serialize it using that same
+                    // vector - we can't simplify to some theoretically parallel axis-aligned
+                    // vector.
+                    None
+                } else if v.is_parallel_to(&DirectionVector::new(1., 0., 0.)) {
+                    Some("x ")
                 } else if v.is_parallel_to(&DirectionVector::new(0., 1., 0.)) {
-                    dest.write_char('y')?;
+                    Some("y ")
                 } else if v.is_parallel_to(&DirectionVector::new(0., 0., 1.)) {
-                    dest.write_char('z')?;
+                    // When we're parallel to the z-axis, we can just serialize the angle.
+                    return angle.to_css(dest);
                 } else {
-                    x.to_css(dest)?;
-                    dest.write_char(' ')?;
-                    y.to_css(dest)?;
-                    dest.write_char(' ')?;
-                    z.to_css(dest)?;
+                    None
+                };
+                match axis {
+                    Some(a) => dest.write_str(a)?,
+                    None => {
+                        x.to_css(dest)?;
+                        dest.write_char(' ')?;
+                        y.to_css(dest)?;
+                        dest.write_char(' ')?;
+                        z.to_css(dest)?;
+                        dest.write_char(' ')?;
+                    }
                 }
-                dest.write_char(' ')?;
                 angle.to_css(dest)
             },
         }
diff --git a/components/style/values/mod.rs b/components/style/values/mod.rs
index 79ec424ab3a..b8e7f1f3b8b 100644
--- a/components/style/values/mod.rs
+++ b/components/style/values/mod.rs
@@ -464,29 +464,34 @@ impl ToCss for CustomIdent {
     }
 }
 
+/// The <timeline-name> or <keyframes-name>.
+/// The definition of these two names are the same, so we use the same type for them.
+///
+/// <https://drafts.csswg.org/css-animations-2/#typedef-timeline-name>
 /// <https://drafts.csswg.org/css-animations/#typedef-keyframes-name>
 #[derive(
     Clone, Debug, MallocSizeOf, SpecifiedValueInfo, ToComputedValue, ToResolvedValue, ToShmem,
 )]
-pub enum KeyframesName {
+#[repr(C, u8)]
+pub enum TimelineOrKeyframesName {
     /// <custom-ident>
     Ident(CustomIdent),
     /// <string>
     QuotedString(Atom),
 }
 
-impl KeyframesName {
+impl TimelineOrKeyframesName {
     /// <https://drafts.csswg.org/css-animations/#dom-csskeyframesrule-name>
     pub fn from_ident(value: &str) -> Self {
         let location = SourceLocation { line: 0, column: 0 };
         let custom_ident = CustomIdent::from_ident(location, &value.into(), &["none"]).ok();
         match custom_ident {
-            Some(ident) => KeyframesName::Ident(ident),
-            None => KeyframesName::QuotedString(value.into()),
+            Some(ident) => Self::Ident(ident),
+            None => Self::QuotedString(value.into()),
         }
     }
 
-    /// Create a new KeyframesName from Atom.
+    /// Create a new TimelineOrKeyframesName from Atom.
     #[cfg(feature = "gecko")]
     pub fn from_atom(atom: Atom) -> Self {
         debug_assert_ne!(atom, atom!(""));
@@ -494,19 +499,19 @@ impl KeyframesName {
         // FIXME: We might want to preserve <string>, but currently Gecko
         // stores both of <custom-ident> and <string> into nsAtom, so
         // we can't tell it.
-        KeyframesName::Ident(CustomIdent(atom))
+        Self::Ident(CustomIdent(atom))
     }
 
     /// The name as an Atom
     pub fn as_atom(&self) -> &Atom {
         match *self {
-            KeyframesName::Ident(ref ident) => &ident.0,
-            KeyframesName::QuotedString(ref atom) => atom,
+            Self::Ident(ref ident) => &ident.0,
+            Self::QuotedString(ref atom) => atom,
         }
     }
 }
 
-impl Eq for KeyframesName {}
+impl Eq for TimelineOrKeyframesName {}
 
 /// A trait that returns whether a given type is the `auto` value or not. So far
 /// only needed for background-size serialization, which special-cases `auto`.
@@ -515,13 +520,13 @@ pub trait IsAuto {
     fn is_auto(&self) -> bool;
 }
 
-impl PartialEq for KeyframesName {
+impl PartialEq for TimelineOrKeyframesName {
     fn eq(&self, other: &Self) -> bool {
         self.as_atom() == other.as_atom()
     }
 }
 
-impl hash::Hash for KeyframesName {
+impl hash::Hash for TimelineOrKeyframesName {
     fn hash<H>(&self, state: &mut H)
     where
         H: hash::Hasher,
@@ -530,32 +535,40 @@ impl hash::Hash for KeyframesName {
     }
 }
 
-impl Parse for KeyframesName {
+impl Parse for TimelineOrKeyframesName {
     fn parse<'i, 't>(
         _context: &ParserContext,
         input: &mut Parser<'i, 't>,
     ) -> Result<Self, ParseError<'i>> {
         let location = input.current_source_location();
         match *input.next()? {
-            Token::Ident(ref s) => Ok(KeyframesName::Ident(CustomIdent::from_ident(
+            Token::Ident(ref s) => Ok(Self::Ident(CustomIdent::from_ident(
                 location,
                 s,
                 &["none"],
             )?)),
-            Token::QuotedString(ref s) => Ok(KeyframesName::QuotedString(Atom::from(s.as_ref()))),
+            Token::QuotedString(ref s) => {
+                Ok(Self::QuotedString(Atom::from(s.as_ref())))
+            },
             ref t => Err(location.new_unexpected_token_error(t.clone())),
         }
     }
 }
 
-impl ToCss for KeyframesName {
+impl ToCss for TimelineOrKeyframesName {
     fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
     where
         W: Write,
     {
         match *self {
-            KeyframesName::Ident(ref ident) => ident.to_css(dest),
-            KeyframesName::QuotedString(ref atom) => atom.to_string().to_css(dest),
+            Self::Ident(ref ident) => ident.to_css(dest),
+            Self::QuotedString(ref atom) => atom.to_string().to_css(dest),
         }
     }
 }
+
+/// The typedef of <timeline-name>.
+pub type TimelineName = TimelineOrKeyframesName;
+
+/// The typedef of <keyframes-name>.
+pub type KeyframesName = TimelineOrKeyframesName;
diff --git a/components/style/values/specified/angle.rs b/components/style/values/specified/angle.rs
index fa60f66507d..08c6b443678 100644
--- a/components/style/values/specified/angle.rs
+++ b/components/style/values/specified/angle.rs
@@ -130,6 +130,15 @@ impl Angle {
         }
     }
 
+    /// Creates an angle with the given value in radians.
+    #[inline]
+    pub fn from_radians(value: CSSFloat) -> Self {
+        Angle {
+            value: AngleDimension::Rad(value),
+            was_calc: false,
+        }
+    }
+
     /// Return `0deg`.
     pub fn zero() -> Self {
         Self::from_degrees(0.0, false)
@@ -141,6 +150,13 @@ impl Angle {
         self.value.degrees()
     }
 
+    /// Returns the value of the angle in radians.
+    #[inline]
+    pub fn radians(&self) -> CSSFloat {
+        const RAD_PER_DEG: f32 = PI / 180.0;
+        self.value.degrees() * RAD_PER_DEG
+    }
+
     /// Whether this specified angle came from a `calc()` expression.
     #[inline]
     pub fn was_calc(&self) -> bool {
diff --git a/components/style/values/specified/box.rs b/components/style/values/specified/box.rs
index 76dd263e831..7d5543cf7fc 100644
--- a/components/style/values/specified/box.rs
+++ b/components/style/values/specified/box.rs
@@ -13,7 +13,7 @@ use crate::values::generics::box_::Perspective as GenericPerspective;
 use crate::values::generics::box_::{GenericVerticalAlign, VerticalAlignKeyword};
 use crate::values::specified::length::{LengthPercentage, NonNegativeLength};
 use crate::values::specified::{AllowQuirks, Number};
-use crate::values::{CustomIdent, KeyframesName};
+use crate::values::{CustomIdent, KeyframesName, TimelineName};
 use crate::Atom;
 use cssparser::Parser;
 use num_traits::FromPrimitive;
@@ -110,8 +110,6 @@ pub enum DisplayInside {
     #[cfg(feature = "gecko")]
     MozBox,
     #[cfg(feature = "gecko")]
-    MozStack,
-    #[cfg(feature = "gecko")]
     MozDeck,
     #[cfg(feature = "gecko")]
     MozPopup,
@@ -226,8 +224,6 @@ impl Display {
     #[cfg(feature = "gecko")]
     pub const MozInlineBox: Self = Self::new(DisplayOutside::Inline, DisplayInside::MozBox);
     #[cfg(feature = "gecko")]
-    pub const MozStack: Self = Self::new(DisplayOutside::XUL, DisplayInside::MozStack);
-    #[cfg(feature = "gecko")]
     pub const MozDeck: Self = Self::new(DisplayOutside::XUL, DisplayInside::MozDeck);
     #[cfg(feature = "gecko")]
     pub const MozPopup: Self = Self::new(DisplayOutside::XUL, DisplayInside::MozPopup);
@@ -608,8 +604,6 @@ impl Parse for Display {
             #[cfg(feature = "gecko")]
             "-moz-inline-box" if moz_box_display_values_enabled(context) => Display::MozInlineBox,
             #[cfg(feature = "gecko")]
-            "-moz-stack" if moz_display_values_enabled(context) => Display::MozStack,
-            #[cfg(feature = "gecko")]
             "-moz-deck" if moz_display_values_enabled(context) => Display::MozDeck,
             #[cfg(feature = "gecko")]
             "-moz-popup" if moz_display_values_enabled(context) => Display::MozPopup,
@@ -768,6 +762,67 @@ impl Parse for AnimationName {
     }
 }
 
+/// A value for the <single-animation-timeline>.
+///
+/// https://drafts.csswg.org/css-animations-2/#typedef-single-animation-timeline
+/// cbindgen:private-default-tagged-enum-constructor=false
+#[derive(
+    Clone,
+    Debug,
+    Eq,
+    Hash,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(C, u8)]
+pub enum AnimationTimeline {
+    /// Use default timeline. The animation’s timeline is a DocumentTimeline.
+    Auto,
+    /// The animation is not associated with a timeline.
+    None,
+    /// The scroll-timeline name
+    Timeline(TimelineName),
+}
+
+impl AnimationTimeline {
+    /// Returns the `auto` value.
+    pub fn auto() -> Self {
+        Self::Auto
+    }
+
+    /// Returns true if it is auto (i.e. the default value).
+    pub fn is_auto(&self) -> bool {
+        matches!(self, Self::Auto)
+    }
+}
+
+impl Parse for AnimationTimeline {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        // We are using the same parser for TimelineName and KeyframesName, but animation-timeline
+        // accepts "auto", so need to manually parse this. (We can not derive Parse because
+        // TimelineName excludes only "none" keyword.)
+        // FIXME: Bug 1733260: we may drop None based on the spec issue:
+        // Note: https://github.com/w3c/csswg-drafts/issues/6674.
+        if input.try_parse(|i| i.expect_ident_matching("auto")).is_ok() {
+            return Ok(Self::Auto);
+        }
+
+        if input.try_parse(|i| i.expect_ident_matching("none")).is_ok() {
+            return Ok(Self::None);
+        }
+
+        TimelineName::parse(context, input).map(AnimationTimeline::Timeline)
+    }
+}
+
 /// https://drafts.csswg.org/css-scroll-snap-1/#snap-axis
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
diff --git a/components/style/values/specified/calc.rs b/components/style/values/specified/calc.rs
index a22d6fd5f18..a043e4ea96e 100644
--- a/components/style/values/specified/calc.rs
+++ b/components/style/values/specified/calc.rs
@@ -21,7 +21,7 @@ use style_traits::values::specified::AllowedNumericType;
 use style_traits::{CssWriter, ParseError, SpecifiedValueInfo, StyleParseErrorKind, ToCss};
 
 /// The name of the mathematical function that we're parsing.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, Parse)]
 pub enum MathFunction {
     /// `calc()`: https://drafts.csswg.org/css-values-4/#funcdef-calc
     Calc,
@@ -31,6 +31,18 @@ pub enum MathFunction {
     Max,
     /// `clamp()`: https://drafts.csswg.org/css-values-4/#funcdef-clamp
     Clamp,
+    /// `sin()`: https://drafts.csswg.org/css-values-4/#funcdef-sin
+    Sin,
+    /// `cos()`: https://drafts.csswg.org/css-values-4/#funcdef-cos
+    Cos,
+    /// `tan()`: https://drafts.csswg.org/css-values-4/#funcdef-tan
+    Tan,
+    /// `asin()`: https://drafts.csswg.org/css-values-4/#funcdef-asin
+    Asin,
+    /// `acos()`: https://drafts.csswg.org/css-values-4/#funcdef-acos
+    Acos,
+    /// `atan()`: https://drafts.csswg.org/css-values-4/#funcdef-atan
+    Atan,
 }
 
 /// A leaf node inside a `Calc` expression's AST.
@@ -231,6 +243,13 @@ impl generic::CalcNodeLeaf for Leaf {
     }
 }
 
+fn trig_enabled() -> bool {
+    #[cfg(feature = "gecko")]
+    return static_prefs::pref!("layout.css.trig.enabled");
+    #[cfg(feature = "servo")]
+    return false;
+}
+
 /// A calc node representation for specified values.
 pub type CalcNode = generic::GenericCalcNode<Leaf>;
 
@@ -301,6 +320,17 @@ impl CalcNode {
                 let function = CalcNode::math_function(name, location)?;
                 CalcNode::parse(context, input, function, expected_unit)
             },
+            (&Token::Ident(ref ident), _) => {
+                if !trig_enabled() {
+                    return Err(location.new_unexpected_token_error(Token::Ident(ident.clone())));
+                }
+                let number = match_ignore_ascii_case! { &**ident,
+                    "e" => std::f32::consts::E,
+                    "pi" => std::f32::consts::PI,
+                    _ => return Err(location.new_unexpected_token_error(Token::Ident(ident.clone()))),
+                };
+                Ok(CalcNode::Leaf(Leaf::Number(number)))
+            },
             (t, _) => Err(location.new_unexpected_token_error(t.clone())),
         }
     }
@@ -350,6 +380,47 @@ impl CalcNode {
 
                     Ok(Self::MinMax(arguments.into(), op))
                 },
+                MathFunction::Sin |
+                MathFunction::Cos |
+                MathFunction::Tan => {
+                    let argument = Self::parse_argument(context, input, CalcUnit::Angle)?;
+                    let radians = match argument.to_number() {
+                        Ok(v) => v,
+                        Err(()) => match argument.to_angle() {
+                            Ok(angle) => angle.radians(),
+                            Err(()) => return Err(
+                                input.new_custom_error(StyleParseErrorKind::UnspecifiedError)
+                            ),
+                        },
+                    };
+                    let number = match function {
+                        MathFunction::Sin => radians.sin(),
+                        MathFunction::Cos => radians.cos(),
+                        MathFunction::Tan => radians.tan(),
+                        _ => unsafe { debug_unreachable!("We just checked!"); },
+                    };
+                    Ok(Self::Leaf(Leaf::Number(number)))
+                },
+                MathFunction::Asin |
+                MathFunction::Acos |
+                MathFunction::Atan => {
+                    let argument = Self::parse_argument(context, input, CalcUnit::Number)?;
+                    let number = match argument.to_number() {
+                        Ok(v) => v,
+                        Err(()) => return Err(
+                            input.new_custom_error(StyleParseErrorKind::UnspecifiedError)
+                        ),
+                    };
+
+                    let radians = match function {
+                        MathFunction::Asin => number.asin(),
+                        MathFunction::Acos => number.acos(),
+                        MathFunction::Atan => number.atan(),
+                        _ => unsafe { debug_unreachable!("We just checked!"); },
+                    };
+
+                    Ok(Self::Leaf(Leaf::Angle(Angle::from_radians(radians))))
+                },
             }
         })
     }
@@ -522,13 +593,18 @@ impl CalcNode {
         name: &CowRcStr<'i>,
         location: cssparser::SourceLocation,
     ) -> Result<MathFunction, ParseError<'i>> {
-        Ok(match_ignore_ascii_case! { &*name,
-            "calc" => MathFunction::Calc,
-            "min" => MathFunction::Min,
-            "max" => MathFunction::Max,
-            "clamp" => MathFunction::Clamp,
-            _ => return Err(location.new_unexpected_token_error(Token::Function(name.clone()))),
-        })
+        use self::MathFunction::*;
+
+        let function = match MathFunction::from_ident(&*name) {
+            Ok(f) => f,
+            Err(()) => return Err(location.new_unexpected_token_error(Token::Function(name.clone()))),
+        };
+
+        if matches!(function, Sin | Cos | Tan | Asin | Acos | Atan) && !trig_enabled() {
+            return Err(location.new_unexpected_token_error(Token::Function(name.clone())));
+        }
+
+        Ok(function)
     }
 
     /// Convenience parsing function for integers.
diff --git a/components/style/values/specified/color.rs b/components/style/values/specified/color.rs
index f84883702c7..fe22266ab23 100644
--- a/components/style/values/specified/color.rs
+++ b/components/style/values/specified/color.rs
@@ -194,20 +194,6 @@ impl ToCss for ColorMix {
     }
 }
 
-/// The color scheme for a specific system color.
-#[cfg(feature = "gecko")]
-#[derive(Clone, Copy, Debug, MallocSizeOf, Parse, PartialEq, ToCss, ToShmem)]
-#[repr(u8)]
-pub enum SystemColorScheme {
-    /// The default color-scheme for the document.
-    #[css(skip)]
-    Default,
-    /// A light color scheme.
-    Light,
-    /// A dark color scheme.
-    Dark,
-}
-
 /// Specified color value
 #[derive(Clone, Debug, MallocSizeOf, PartialEq, ToShmem)]
 pub enum Color {
@@ -222,10 +208,9 @@ pub enum Color {
     },
     /// A complex color value from computed value
     Complex(ComputedColor),
-    /// Either a system color, or a `-moz-system-color(<system-color>, light|dark)`
-    /// function which allows chrome code to choose between color schemes.
+    /// A system color.
     #[cfg(feature = "gecko")]
-    System(SystemColor, SystemColorScheme),
+    System(SystemColor),
     /// A color mix.
     ColorMix(Box<ColorMix>),
     /// Quirksmode-only rule for inheriting color from the body
@@ -233,36 +218,19 @@ pub enum Color {
     InheritFromBodyQuirk,
 }
 
-/// System colors.
+/// System colors. A bunch of these are ad-hoc, others come from Windows:
+///
+///   https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getsyscolor
+///
+/// Others are HTML/CSS specific. Spec is:
+///
+///   https://drafts.csswg.org/css-color/#css-system-colors
+///   https://drafts.csswg.org/css-color/#deprecated-system-colors
 #[allow(missing_docs)]
 #[cfg(feature = "gecko")]
 #[derive(Clone, Copy, Debug, MallocSizeOf, Parse, PartialEq, ToCss, ToShmem)]
 #[repr(u8)]
 pub enum SystemColor {
-    #[css(skip)]
-    WindowBackground,
-    #[css(skip)]
-    WindowForeground,
-    #[css(skip)]
-    WidgetBackground,
-    #[css(skip)]
-    WidgetForeground,
-    #[css(skip)]
-    WidgetSelectBackground,
-    #[css(skip)]
-    WidgetSelectForeground,
-    #[css(skip)]
-    Widget3DHighlight,
-    #[css(skip)]
-    Widget3DShadow,
-    #[css(skip)]
-    TextBackground,
-    #[css(skip)]
-    TextForeground,
-    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
-    TextSelectBackground,
-    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
-    TextSelectForeground,
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     TextSelectBackgroundDisabled,
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
@@ -310,6 +278,7 @@ pub enum SystemColor {
     #[css(skip)]
     ThemedScrollbarThumbInactive,
     Activeborder,
+    /// Background in the (active) titlebar.
     Activecaption,
     Appworkspace,
     Background,
@@ -317,16 +286,23 @@ pub enum SystemColor {
     Buttonhighlight,
     Buttonshadow,
     Buttontext,
+    /// Text color in the (active) titlebar.
     Captiontext,
     #[parse(aliases = "-moz-field")]
     Field,
+    /// Used for disabled field backgrounds.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
+    MozDisabledfield,
     #[parse(aliases = "-moz-fieldtext")]
     Fieldtext,
+
     Graytext,
     Highlight,
     Highlighttext,
     Inactiveborder,
+    /// Background in the (inactive) titlebar.
     Inactivecaption,
+    /// Text color in the (inactive) titlebar.
     Inactivecaptiontext,
     Infobackground,
     Infotext,
@@ -351,13 +327,15 @@ pub enum SystemColor {
     /// Used to highlight valid regions to drop something onto.
     MozDragtargetzone,
     /// Used for selected but not focused cell backgrounds.
+    #[parse(aliases = "-moz-html-cellhighlight")]
     MozCellhighlight,
     /// Used for selected but not focused cell text.
+    #[parse(aliases = "-moz-html-cellhighlighttext")]
     MozCellhighlighttext,
-    /// Used for selected but not focused html cell backgrounds.
-    MozHtmlCellhighlight,
-    /// Used for selected but not focused html cell text.
-    MozHtmlCellhighlighttext,
+    /// Used for selected and focused html cell backgrounds.
+    Selecteditem,
+    /// Used for selected and focused html cell text.
+    Selecteditemtext,
     /// Used to button text background when hovered.
     MozButtonhoverface,
     /// Used to button text color when hovered.
@@ -378,10 +356,16 @@ pub enum SystemColor {
 
     /// Used for button text when pressed.
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
-    MozGtkButtonactivetext,
+    MozButtonactivetext,
+
+    /// Used for button background when pressed.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
+    MozButtonactiveface,
+
+    /// Used for button background when disabled.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
+    MozButtondisabledface,
 
-    /// Used for button text when pressed.
-    MozMacButtonactivetext,
     /// Background color of chrome toolbars in active windows.
     MozMacChromeActive,
     /// Background color of chrome toolbars in inactive windows.
@@ -423,6 +407,10 @@ pub enum SystemColor {
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     MozAccentColorForeground,
 
+    /// The background-color for :autofill-ed inputs.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
+    MozAutofillBackground,
+
     /// Media rebar text.
     MozWinMediatext,
     /// Communications rebar text.
@@ -457,14 +445,6 @@ pub enum SystemColor {
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     MozColheaderhovertext,
 
-    /// Color of text in the (active) titlebar.
-    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
-    MozGtkTitlebarText,
-
-    /// Color of text in the (inactive) titlebar.
-    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
-    MozGtkTitlebarInactiveText,
-
     #[css(skip)]
     End, // Just for array-indexing purposes.
 }
@@ -472,31 +452,20 @@ pub enum SystemColor {
 #[cfg(feature = "gecko")]
 impl SystemColor {
     #[inline]
-    fn compute(&self, cx: &Context, scheme: SystemColorScheme) -> ComputedColor {
+    fn compute(&self, cx: &Context) -> ComputedColor {
         use crate::gecko_bindings::bindings;
 
-        let colors = &cx.device().pref_sheet_prefs().mColors;
-        let style_color_scheme = cx.style().get_inherited_ui().clone_color_scheme();
-
-        // TODO: At least Canvas / CanvasText should be color-scheme aware
-        // (probably the link colors too).
-        convert_nscolor_to_computedcolor(match *self {
-            SystemColor::Canvastext => colors.mDefault,
-            SystemColor::Canvas => colors.mDefaultBackground,
-            SystemColor::Linktext => colors.mLink,
-            SystemColor::Activetext => colors.mActiveLink,
-            SystemColor::Visitedtext => colors.mVisitedLink,
-
-            _ => {
-                let color = unsafe {
-                    bindings::Gecko_GetLookAndFeelSystemColor(*self as i32, cx.device().document(), scheme, &style_color_scheme)
-                };
-                if color == bindings::NS_SAME_AS_FOREGROUND_COLOR {
-                    return ComputedColor::currentcolor();
-                }
-                color
-            },
-        })
+        // TODO: We should avoid cloning here most likely, though it's
+        // cheap-ish.
+        let style_color_scheme =
+            cx.style().get_inherited_ui().clone_color_scheme();
+        let color = unsafe {
+            bindings::Gecko_ComputeSystemColor(*self, cx.device().document(), &style_color_scheme)
+        };
+        if color == bindings::NS_SAME_AS_FOREGROUND_COLOR {
+            return ComputedColor::currentcolor();
+        }
+        convert_nscolor_to_computedcolor(color)
     }
 }
 
@@ -570,21 +539,6 @@ impl<'a, 'b: 'a, 'i: 'a> ::cssparser::ColorComponentParser<'i> for ColorComponen
     }
 }
 
-#[cfg(feature = "gecko")]
-fn parse_moz_system_color<'i, 't>(
-    context: &ParserContext,
-    input: &mut Parser<'i, 't>,
-) -> Result<(SystemColor, SystemColorScheme), ParseError<'i>> {
-    debug_assert!(context.chrome_rules_enabled());
-    input.expect_function_matching("-moz-system-color")?;
-    input.parse_nested_block(|input| {
-        let color = SystemColor::parse(context, input)?;
-        input.expect_comma()?;
-        let scheme = SystemColorScheme::parse(input)?;
-        Ok((color, scheme))
-    })
-}
-
 impl Parse for Color {
     fn parse<'i, 't>(
         context: &ParserContext,
@@ -610,15 +564,7 @@ impl Parse for Color {
                 #[cfg(feature = "gecko")]
                 {
                     if let Ok(system) = input.try_parse(|i| SystemColor::parse(context, i)) {
-                        return Ok(Color::System(system, SystemColorScheme::Default));
-                    }
-
-                    if context.chrome_rules_enabled() {
-                        if let Ok((color, scheme)) =
-                            input.try_parse(|i| parse_moz_system_color(context, i))
-                        {
-                            return Ok(Color::System(color, scheme));
-                        }
+                        return Ok(Color::System(system));
                     }
                 }
 
@@ -657,17 +603,7 @@ impl ToCss for Color {
             Color::Complex(_) => Ok(()),
             Color::ColorMix(ref mix) => mix.to_css(dest),
             #[cfg(feature = "gecko")]
-            Color::System(system, scheme) => {
-                if scheme == SystemColorScheme::Default {
-                    system.to_css(dest)
-                } else {
-                    dest.write_str("-moz-system-color(")?;
-                    system.to_css(dest)?;
-                    dest.write_str(", ")?;
-                    scheme.to_css(dest)?;
-                    dest.write_char(')')
-                }
-            },
+            Color::System(system) => system.to_css(dest),
             #[cfg(feature = "gecko")]
             Color::InheritFromBodyQuirk => Ok(()),
         }
@@ -835,7 +771,7 @@ impl Color {
                 ))
             },
             #[cfg(feature = "gecko")]
-            Color::System(system, scheme) => system.compute(context?, scheme),
+            Color::System(system) => system.compute(context?),
             #[cfg(feature = "gecko")]
             Color::InheritFromBodyQuirk => ComputedColor::rgba(context?.device().body_text_color()),
         })
@@ -995,6 +931,11 @@ impl ColorScheme {
             bits: ColorSchemeFlags::empty(),
         }
     }
+
+    /// Returns the raw bitfield.
+    pub fn raw_bits(&self) -> u8 {
+        self.bits.bits
+    }
 }
 
 impl Parse for ColorScheme {
diff --git a/components/style/values/specified/gecko.rs b/components/style/values/specified/gecko.rs
index 3e3085c8849..9b01cc2a6c4 100644
--- a/components/style/values/specified/gecko.rs
+++ b/components/style/values/specified/gecko.rs
@@ -49,6 +49,11 @@ impl Parse for IntersectionObserverRootMargin {
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
     ) -> Result<Self, ParseError<'i>> {
+        use crate::Zero;
+        if input.is_exhausted() {
+            // If there are zero elements in tokens, set tokens to ["0px"].
+            return Ok(IntersectionObserverRootMargin(Rect::all(LengthPercentage::zero())));
+        }
         let rect = Rect::parse_with(context, input, parse_pixel_or_percent)?;
         Ok(IntersectionObserverRootMargin(rect))
     }
diff --git a/components/style/values/specified/image.rs b/components/style/values/specified/image.rs
index f2167e52800..095ce20f4c7 100644
--- a/components/style/values/specified/image.rs
+++ b/components/style/values/specified/image.rs
@@ -1226,3 +1226,42 @@ impl MozImageRect {
         })
     }
 }
+
+/// https://drafts.csswg.org/css-images/#propdef-image-rendering
+#[allow(missing_docs)]
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    Hash,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToCss,
+    ToComputedValue,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[repr(u8)]
+pub enum ImageRendering {
+    Auto,
+    #[cfg(feature = "gecko")]
+    Smooth,
+    #[parse(aliases = "-moz-crisp-edges")]
+    CrispEdges,
+    Pixelated,
+    // From the spec:
+    //
+    //     This property previously accepted the values optimizeSpeed and
+    //     optimizeQuality. These are now deprecated; a user agent must accept
+    //     them as valid values but must treat them as having the same behavior
+    //     as crisp-edges and smooth respectively, and authors must not use
+    //     them.
+    //
+    #[cfg(feature = "gecko")]
+    Optimizespeed,
+    #[cfg(feature = "gecko")]
+    Optimizequality,
+}
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index e73a9ddfde0..6105155d81d 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -1235,7 +1235,7 @@ macro_rules! parse_size_non_length {
                 #[cfg(feature = "gecko")]
                 "max-content" | "-moz-max-content" => $size::MaxContent,
                 #[cfg(feature = "gecko")]
-                "-moz-fit-content" => $size::MozFitContent,
+                "fit-content" | "-moz-fit-content" => $size::FitContent,
                 #[cfg(feature = "gecko")]
                 "-moz-available" => $size::MozAvailable,
                 $auto_or_none => $size::$auto_or_none_ident,
diff --git a/components/style/values/specified/mod.rs b/components/style/values/specified/mod.rs
index 6cd952cd82f..64bc063960e 100644
--- a/components/style/values/specified/mod.rs
+++ b/components/style/values/specified/mod.rs
@@ -36,7 +36,7 @@ pub use self::basic_shape::FillRule;
 pub use self::border::{BorderCornerRadius, BorderImageSlice, BorderImageWidth};
 pub use self::border::{BorderImageRepeat, BorderImageSideWidth};
 pub use self::border::{BorderRadius, BorderSideWidth, BorderSpacing, BorderStyle};
-pub use self::box_::{AnimationIterationCount, AnimationName, Contain, Display};
+pub use self::box_::{AnimationIterationCount, AnimationName, AnimationTimeline, Contain, Display};
 pub use self::box_::{Appearance, BreakBetween, BreakWithin};
 pub use self::box_::{Clear, Float, Overflow, OverflowAnchor};
 pub use self::box_::{OverflowClipBox, OverscrollBehavior, Perspective, Resize};
@@ -55,7 +55,7 @@ pub use self::font::{FontVariantAlternates, FontWeight};
 pub use self::font::{FontVariantEastAsian, FontVariationSettings};
 pub use self::font::{MathDepth, MozScriptMinSize, MozScriptSizeMultiplier, XLang, XTextZoom};
 pub use self::image::{EndingShape as GradientEndingShape, Gradient};
-pub use self::image::{Image, MozImageRect};
+pub use self::image::{Image, MozImageRect, ImageRendering};
 pub use self::length::{AbsoluteLength, CalcLengthPercentage, CharacterWidth};
 pub use self::length::{FontRelativeLength, Length, LengthOrNumber, NonNegativeLengthOrNumber};
 pub use self::length::{LengthOrAuto, LengthPercentage, LengthPercentageOrAuto};
diff --git a/servo-tidy.toml b/servo-tidy.toml
index da2ca334ac9..b22afbf5ecf 100644
--- a/servo-tidy.toml
+++ b/servo-tidy.toml
@@ -82,7 +82,12 @@ files = [
   # These are ignored to avoid diverging from Gecko
   "./components/style/counter_style/mod.rs",
   "./components/style/properties/helpers.mako.rs",
+  "./components/style/rule_collector.rs",
+  "./components/style/selector_map.rs",
+  "./components/style/stylesheets/import_rule.rs",
+  "./components/style/stylesheets/layer_rule.rs",
   "./components/style/stylesheets/rule_parser.rs",
+  "./components/style/stylesheets/scroll_timeline_rule.rs",
   "./components/style/stylist.rs",
   "./components/style/values/computed/font.rs",
   "./components/style/values/computed/image.rs",
diff --git a/tests/unit/style/stylist.rs b/tests/unit/style/stylist.rs
index 31b7869a258..c89a401bbc0 100644
--- a/tests/unit/style/stylist.rs
+++ b/tests/unit/style/stylist.rs
@@ -15,6 +15,7 @@ use style::properties::{PropertyDeclaration, PropertyDeclarationBlock};
 use style::selector_map::SelectorMap;
 use style::selector_parser::{SelectorImpl, SelectorParser};
 use style::shared_lock::SharedRwLock;
+use style::stylesheets::layer_rule::LayerId;
 use style::stylesheets::StyleRule;
 use style::stylist::needs_revalidation_for_testing;
 use style::stylist::{Rule, Stylist};
@@ -52,6 +53,7 @@ fn get_mock_rules(css_selectors: &[&str]) -> (Vec<Vec<Rule>>, SharedRwLock) {
                             AncestorHashes::new(s, QuirksMode::NoQuirks),
                             locked.clone(),
                             i as u32,
+                            LayerId::root(),
                         )
                     })
                     .collect()
diff --git a/tests/wpt/metadata/css/css-transforms/animation/rotate-interpolation.html.ini b/tests/wpt/metadata/css/css-transforms/animation/rotate-interpolation.html.ini
index 8590593db04..75fd404ab49 100644
--- a/tests/wpt/metadata/css/css-transforms/animation/rotate-interpolation.html.ini
+++ b/tests/wpt/metadata/css/css-transforms/animation/rotate-interpolation.html.ini
@@ -344,14 +344,5 @@
   [Web Animations: property <rotate> from [1 -2.5 3.64 100deg\] to [1 -2.5 3.64 -100deg\] at (1) should be [0.22 -0.55 0.8 -100deg\]]
     expected: FAIL
 
-  [CSS Transitions: property <rotate> from [45deg\] to [-1 1 0 60deg\] at (0) should be [45deg\]]
-    expected: FAIL
-
-  [CSS Transitions with transition: all: property <rotate> from [45deg\] to [-1 1 0 60deg\] at (0) should be [45deg\]]
-    expected: FAIL
-
-  [CSS Animations: property <rotate> from [45deg\] to [-1 1 0 60deg\] at (0) should be [45deg\]]
-    expected: FAIL
-
   [Web Animations: property <rotate> from [45deg\] to [-1 1 0 60deg\] at (0) should be [45deg\]]
     expected: FAIL
diff --git a/tests/wpt/metadata/css/css-transforms/parsing/rotate-parsing-valid.html.ini b/tests/wpt/metadata/css/css-transforms/parsing/rotate-parsing-valid.html.ini
deleted file mode 100644
index b7662bdbf26..00000000000
--- a/tests/wpt/metadata/css/css-transforms/parsing/rotate-parsing-valid.html.ini
+++ /dev/null
@@ -1,12 +0,0 @@
-[rotate-parsing-valid.html]
-  [e.style['rotate'\] = "0 0 0 400grad" should set the property value]
-    expected: FAIL
-
-  [e.style['rotate'\] = "400grad z" should set the property value]
-    expected: FAIL
-
-  [e.style['rotate'\] = "0 0 0.5 400grad" should set the property value]
-    expected: FAIL
-
-  [e.style['rotate'\] = "0 0 1 400grad" should set the property value]
-    expected: FAIL
diff --git a/tests/wpt/metadata/css/cssom/shorthand-values.html.ini b/tests/wpt/metadata/css/cssom/shorthand-values.html.ini
index badceaee7fe..a41c085aee4 100644
--- a/tests/wpt/metadata/css/cssom/shorthand-values.html.ini
+++ b/tests/wpt/metadata/css/cssom/shorthand-values.html.ini
@@ -29,8 +29,5 @@
   [The serialization of list-style-type: circle; list-style-position: inside; list-style-image: initial; should be canonical.]
     expected: FAIL
 
-  [The serialization of list-style-type: circle; list-style-position: inside; list-style-image: none; should be canonical.]
-    expected: FAIL
-
   [The serialization of border-top: 1px; border-right: 1px; border-bottom: 1px; border-left: 1px; border-image: none; should be canonical.]
     expected: FAIL