diff --git a/components/layout/animation.rs b/components/layout/animation.rs
index 232f9cd2efe..be6d5baf77c 100644
--- a/components/layout/animation.rs
+++ b/components/layout/animation.rs
@@ -13,6 +13,7 @@ use script_traits::{AnimationState, ConstellationControlMsg, LayoutMsg as Conste
 use std::collections::HashMap;
 use std::sync::mpsc::Receiver;
 use style::animation::{Animation, update_style_for_animation};
+use style::font_metrics::ServoMetricsProvider;
 use style::selector_parser::RestyleDamage;
 use style::timer::Timer;
 
@@ -143,7 +144,8 @@ pub fn recalc_style_for_animations(context: &LayoutContext,
                 let old_style = fragment.style.clone();
                 update_style_for_animation(&context.style_context,
                                            animation,
-                                           &mut fragment.style);
+                                           &mut fragment.style,
+                                           &ServoMetricsProvider);
                 damage |= RestyleDamage::compute(&old_style, &fragment.style);
             }
         }
diff --git a/components/layout/traversal.rs b/components/layout/traversal.rs
index 844a61527b6..88852e4193a 100644
--- a/components/layout/traversal.rs
+++ b/components/layout/traversal.rs
@@ -53,6 +53,7 @@ impl<'a> RecalcStyleAndConstructFlows<'a> {
 impl<'a, E> DomTraversal<E> for RecalcStyleAndConstructFlows<'a>
     where E: TElement,
           E::ConcreteNode: LayoutNode,
+          E::FontMetricsProvider: Send,
 {
     type ThreadLocalContext = ScopedThreadLocalLayoutContext<E>;
 
diff --git a/components/script/layout_wrapper.rs b/components/script/layout_wrapper.rs
index 1956cd6018d..4b118bdf7f1 100644
--- a/components/script/layout_wrapper.rs
+++ b/components/script/layout_wrapper.rs
@@ -67,6 +67,7 @@ use style::data::ElementData;
 use style::dom::{DescendantsBit, DirtyDescendants, LayoutIterator, NodeInfo, OpaqueNode};
 use style::dom::{PresentationalHintsSynthetizer, TElement, TNode, UnsafeNode};
 use style::element_state::*;
+use style::font_metrics::ServoMetricsProvider;
 use style::properties::{ComputedValues, PropertyDeclarationBlock};
 use style::selector_parser::{NonTSPseudoClass, PseudoElement, SelectorImpl};
 use style::shared_lock::{SharedRwLock as StyleSharedRwLock, Locked as StyleLocked};
@@ -373,6 +374,8 @@ impl<'le> PresentationalHintsSynthetizer for ServoLayoutElement<'le> {
 impl<'le> TElement for ServoLayoutElement<'le> {
     type ConcreteNode = ServoLayoutNode<'le>;
 
+    type FontMetricsProvider = ServoMetricsProvider;
+
     fn as_node(&self) -> ServoLayoutNode<'le> {
         ServoLayoutNode::from_layout_js(self.element.upcast())
     }
diff --git a/components/script_layout_interface/wrapper_traits.rs b/components/script_layout_interface/wrapper_traits.rs
index a1c59d83114..88a1dbebb33 100644
--- a/components/script_layout_interface/wrapper_traits.rs
+++ b/components/script_layout_interface/wrapper_traits.rs
@@ -21,6 +21,7 @@ use style::context::SharedStyleContext;
 use style::data::ElementData;
 use style::dom::{LayoutIterator, NodeInfo, PresentationalHintsSynthetizer, TNode};
 use style::dom::OpaqueNode;
+use style::font_metrics::ServoMetricsProvider;
 use style::properties::{CascadeFlags, ServoComputedValues};
 use style::selector_parser::{PseudoElement, PseudoElementCascadeType, SelectorImpl};
 
@@ -411,7 +412,8 @@ pub trait ThreadSafeLayoutElement: Clone + Copy + Sized + Debug +
                                     &context.guards,
                                     &style_pseudo,
                                     Some(data.styles().primary.values()),
-                                    CascadeFlags::empty());
+                                    CascadeFlags::empty(),
+                                    &ServoMetricsProvider);
                             data.styles_mut().cached_pseudos
                                 .insert(style_pseudo.clone(), new_style);
                         }
@@ -426,7 +428,8 @@ pub trait ThreadSafeLayoutElement: Clone + Copy + Sized + Debug +
                                            &context.guards,
                                            unsafe { &self.unsafe_get() },
                                            &style_pseudo,
-                                           data.styles().primary.values());
+                                           data.styles().primary.values(),
+                                           &ServoMetricsProvider);
                             data.styles_mut().cached_pseudos
                                 .insert(style_pseudo.clone(), new_style.unwrap());
                         }
diff --git a/components/style/animation.rs b/components/style/animation.rs
index 6c540d3a708..aeff683f407 100644
--- a/components/style/animation.rs
+++ b/components/style/animation.rs
@@ -10,6 +10,7 @@ use bezier::Bezier;
 use context::SharedStyleContext;
 use dom::{OpaqueNode, UnsafeNode};
 use euclid::point::Point2D;
+use font_metrics::FontMetricsProvider;
 use keyframes::{KeyframesStep, KeyframesStepValue};
 use properties::{self, CascadeFlags, ComputedValues, Importance};
 use properties::animated_properties::{AnimatedProperty, TransitionProperty};
@@ -410,7 +411,8 @@ pub fn start_transitions_if_applicable(new_animations_sender: &Sender<Animation>
 fn compute_style_for_animation_step(context: &SharedStyleContext,
                                     step: &KeyframesStep,
                                     previous_style: &ComputedValues,
-                                    style_from_cascade: &ComputedValues)
+                                    style_from_cascade: &ComputedValues,
+                                    font_metrics_provider: &FontMetricsProvider)
                                     -> ComputedValues {
     match step.value {
         KeyframesStepValue::ComputedValues => style_from_cascade.clone(),
@@ -433,7 +435,7 @@ fn compute_style_for_animation_step(context: &SharedStyleContext,
                                                previous_style,
                                                /* cascade_info = */ None,
                                                &*context.error_reporter,
-                                               /* Metrics provider */ None,
+                                               font_metrics_provider,
                                                CascadeFlags::empty());
             computed
         }
@@ -534,7 +536,8 @@ pub fn update_style_for_animation_frame(mut new_style: &mut Arc<ComputedValues>,
 /// If `damage` is provided, inserts the appropriate restyle damage.
 pub fn update_style_for_animation(context: &SharedStyleContext,
                                   animation: &Animation,
-                                  style: &mut Arc<ComputedValues>) {
+                                  style: &mut Arc<ComputedValues>,
+                                  font_metrics_provider: &FontMetricsProvider) {
     debug!("update_style_for_animation: entering");
     debug_assert!(!animation.is_expired());
 
@@ -658,7 +661,8 @@ pub fn update_style_for_animation(context: &SharedStyleContext,
             let from_style = compute_style_for_animation_step(context,
                                                               last_keyframe,
                                                               &**style,
-                                                              &state.cascade_style);
+                                                              &state.cascade_style,
+                                                              font_metrics_provider);
 
             // NB: The spec says that the timing function can be overwritten
             // from the keyframe style.
@@ -672,7 +676,8 @@ pub fn update_style_for_animation(context: &SharedStyleContext,
             let target_style = compute_style_for_animation_step(context,
                                                                 target_keyframe,
                                                                 &from_style,
-                                                                &state.cascade_style);
+                                                                &state.cascade_style,
+                                                                font_metrics_provider);
 
             let mut new_style = (*style).clone();
 
diff --git a/components/style/build_gecko.rs b/components/style/build_gecko.rs
index deffa87ce96..85997cdb3c2 100644
--- a/components/style/build_gecko.rs
+++ b/components/style/build_gecko.rs
@@ -308,6 +308,7 @@ mod bindings {
             "mozilla::SERVO_PREF_.*",
             "kNameSpaceID_.*",
             "kGenericFont_.*",
+            "kPresContext_.*",
         ];
         let whitelist = [
             "RawGecko.*",
@@ -338,6 +339,7 @@ mod bindings {
             "FontFamilyListRefCnt",
             "FontFamilyName",
             "FontFamilyType",
+            "FontSizePrefs",
             "FragmentOrURL",
             "FrameRequestCallback",
             "GeckoParserExtraData",
@@ -623,6 +625,7 @@ mod bindings {
             "ComputedTimingFunction_BeforeFlag",
             "FontFamilyList",
             "FontFamilyType",
+            "FontSizePrefs",
             "Keyframe",
             "ServoBundledURI",
             "ServoElementSnapshot",
diff --git a/components/style/context.rs b/components/style/context.rs
index 8d9871604a0..583b5bda187 100644
--- a/components/style/context.rs
+++ b/components/style/context.rs
@@ -12,6 +12,7 @@ use data::ElementData;
 use dom::{OpaqueNode, TNode, TElement, SendElement};
 use error_reporting::ParseErrorReporter;
 use euclid::Size2D;
+use font_metrics::FontMetricsProvider;
 #[cfg(feature = "gecko")] use gecko_bindings::structs;
 use matching::StyleSharingCandidateCache;
 use parking_lot::RwLock;
@@ -291,6 +292,9 @@ pub struct ThreadLocalStyleContext<E: TElement> {
     pub statistics: TraversalStatistics,
     /// Information related to the current element, non-None during processing.
     pub current_element_info: Option<CurrentElementInfo>,
+    /// The struct used to compute and cache font metrics from style
+    /// for evaluation of the font-relative em/ch units and font-size
+    pub font_metrics_provider: E::FontMetricsProvider,
 }
 
 impl<E: TElement> ThreadLocalStyleContext<E> {
@@ -303,6 +307,7 @@ impl<E: TElement> ThreadLocalStyleContext<E> {
             tasks: Vec::new(),
             statistics: TraversalStatistics::default(),
             current_element_info: None,
+            font_metrics_provider: E::FontMetricsProvider::create_from(shared),
         }
     }
 
diff --git a/components/style/dom.rs b/components/style/dom.rs
index bd201683afe..f64c594bd4e 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -12,6 +12,7 @@ use atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
 #[cfg(feature = "gecko")] use context::UpdateAnimationsTasks;
 use data::ElementData;
 use element_state::ElementState;
+use font_metrics::FontMetricsProvider;
 use properties::{ComputedValues, PropertyDeclarationBlock};
 use selector_parser::{ElementExt, PreExistingComputedValues, PseudoElement};
 use selectors::matching::ElementSelectorFlags;
@@ -278,6 +279,12 @@ pub trait TElement : PartialEq + Debug + Sized + Copy + Clone + ElementExt + Pre
     /// The concrete node type.
     type ConcreteNode: TNode<ConcreteElement = Self>;
 
+    /// Type of the font metrics provider
+    ///
+    /// XXXManishearth It would be better to make this a type parameter on
+    /// ThreadLocalStyleContext and StyleContext
+    type FontMetricsProvider: FontMetricsProvider;
+
     /// Get this element as a node.
     fn as_node(&self) -> Self::ConcreteNode;
 
diff --git a/components/style/font_metrics.rs b/components/style/font_metrics.rs
index 417c7999845..5210ded345e 100644
--- a/components/style/font_metrics.rs
+++ b/components/style/font_metrics.rs
@@ -8,6 +8,7 @@
 
 use Atom;
 use app_units::Au;
+use context::SharedStyleContext;
 use euclid::Size2D;
 use std::fmt;
 
@@ -31,8 +32,44 @@ pub enum FontMetricsQueryResult {
     NotAvailable,
 }
 
+// TODO: Servo's font metrics provider will probably not live in this crate, so this will
+// have to be replaced with something else (perhaps a trait method on TElement)
+// when we get there
+#[derive(Debug)]
+#[cfg(feature = "servo")]
+/// Dummy metrics provider for Servo. Knows nothing about fonts and does not provide
+/// any metrics.
+pub struct ServoMetricsProvider;
+
+#[cfg(feature = "servo")]
+impl FontMetricsProvider for ServoMetricsProvider {
+    fn create_from(_: &SharedStyleContext) -> Self {
+        ServoMetricsProvider
+    }
+
+    fn get_size(&self, _font_name: &Atom, _font_family: u8) -> Au {
+        unreachable!("Dummy provider should never be used to compute font size")
+    }
+}
+
+// Servo's font metrics provider will probably not live in this crate, so this will
+// have to be replaced with something else (perhaps a trait method on TElement)
+// when we get there
+
+#[cfg(feature = "gecko")]
+/// Construct a font metrics provider for the current product
+pub fn get_metrics_provider_for_product() -> ::gecko::wrapper::GeckoFontMetricsProvider {
+    ::gecko::wrapper::GeckoFontMetricsProvider::new()
+}
+
+#[cfg(feature = "servo")]
+/// Construct a font metrics provider for the current product
+pub fn get_metrics_provider_for_product() -> ServoMetricsProvider {
+    ServoMetricsProvider
+}
+
 /// A trait used to represent something capable of providing us font metrics.
-pub trait FontMetricsProvider: Send + Sync + fmt::Debug {
+pub trait FontMetricsProvider: fmt::Debug {
     /// Obtain the metrics for given font family.
     ///
     /// TODO: We could make this take the full list, I guess, and save a few
@@ -41,4 +78,11 @@ pub trait FontMetricsProvider: Send + Sync + fmt::Debug {
     fn query(&self, _font_name: &Atom) -> FontMetricsQueryResult {
         FontMetricsQueryResult::NotAvailable
     }
+
+    /// Get default size of a given language and generic family
+    fn get_size(&self, font_name: &Atom, font_family: u8) -> Au;
+
+    /// Construct from a shared style context
+    fn create_from(context: &SharedStyleContext) -> Self where Self: Sized;
 }
+
diff --git a/components/style/gecko/media_queries.rs b/components/style/gecko/media_queries.rs
index 4b557aa2f8c..6ba63e75d8f 100644
--- a/components/style/gecko/media_queries.rs
+++ b/components/style/gecko/media_queries.rs
@@ -7,6 +7,7 @@
 use app_units::Au;
 use cssparser::{CssStringWriter, Parser, Token};
 use euclid::Size2D;
+use font_metrics::get_metrics_provider_for_product;
 use gecko_bindings::bindings;
 use gecko_bindings::structs::{nsCSSValue, nsCSSUnit, nsStringBuffer};
 use gecko_bindings::structs::{nsMediaExpression_Range, nsMediaFeature};
@@ -499,6 +500,8 @@ impl Expression {
 
         let default_values = device.default_computed_values();
 
+        let provider = get_metrics_provider_for_product();
+
         // http://dev.w3.org/csswg/mediaqueries3/#units
         // em units are relative to the initial font-size.
         let context = computed::Context {
@@ -509,7 +512,7 @@ impl Expression {
             // This cloning business is kind of dumb.... It's because Context
             // insists on having an actual ComputedValues inside itself.
             style: default_values.clone(),
-            font_metrics_provider: None,
+            font_metrics_provider: &provider,
         };
 
         let required_value = match self.value {
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 6f59864b322..e0b1b4af0f0 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -14,13 +14,15 @@
 //! style system it's kind of pointless in the Stylo case, and only Servo forces
 //! the separation between the style system implementation and everything else.
 
+use app_units::Au;
 use atomic_refcell::AtomicRefCell;
-use context::UpdateAnimationsTasks;
+use context::{SharedStyleContext, UpdateAnimationsTasks};
 use data::ElementData;
 use dom::{self, AnimationRules, DescendantsBit, LayoutIterator, NodeInfo, TElement, TNode, UnsafeNode};
 use dom::{OpaqueNode, PresentationalHintsSynthetizer};
 use element_state::ElementState;
 use error_reporting::StdoutErrorReporter;
+use font_metrics::FontMetricsProvider;
 use gecko::global_style_data::GLOBAL_STYLE_DATA;
 use gecko::selector_parser::{SelectorImpl, NonTSPseudoClass, PseudoElement};
 use gecko::snapshot_helpers;
@@ -60,6 +62,7 @@ use selectors::matching::{ElementSelectorFlags, StyleRelations};
 use selectors::parser::{AttrSelector, NamespaceConstraint};
 use shared_lock::Locked;
 use sink::Push;
+use std::cell::RefCell;
 use std::fmt;
 use std::ptr;
 use std::sync::Arc;
@@ -426,8 +429,65 @@ fn get_animation_rule(element: &GeckoElement,
     }
 }
 
+#[derive(Debug)]
+/// Gecko font metrics provider
+pub struct GeckoFontMetricsProvider {
+    /// Cache of base font sizes for each language
+    ///
+    /// Usually will have 1 element.
+    ///
+    // This may be slow on pages using more languages, might be worth optimizing
+    // by caching lang->group mapping separately and/or using a hashmap on larger
+    // loads.
+    pub font_size_cache: RefCell<Vec<(Atom, ::gecko_bindings::structs::FontSizePrefs)>>,
+}
+
+impl GeckoFontMetricsProvider {
+    /// Construct
+    pub fn new() -> Self {
+        GeckoFontMetricsProvider {
+            font_size_cache: RefCell::new(Vec::new()),
+        }
+    }
+}
+
+impl FontMetricsProvider for GeckoFontMetricsProvider {
+    fn create_from(_: &SharedStyleContext) -> GeckoFontMetricsProvider {
+        GeckoFontMetricsProvider::new()
+    }
+
+    fn get_size(&self, font_name: &Atom, font_family: u8) -> Au {
+        use gecko_bindings::bindings::Gecko_GetBaseSize;
+        let mut cache = self.font_size_cache.borrow_mut();
+        if let Some(sizes) = cache.iter().find(|el| el.0 == *font_name) {
+            return sizes.1.size_for_generic(font_family);
+        }
+        let sizes = unsafe {
+            Gecko_GetBaseSize(font_name.as_ptr())
+        };
+        cache.push((font_name.clone(), sizes));
+        sizes.size_for_generic(font_family)
+    }
+}
+
+impl structs::FontSizePrefs {
+    fn size_for_generic(&self, font_family: u8) -> Au {
+        Au(match font_family {
+            structs::kPresContext_DefaultVariableFont_ID => self.mDefaultVariableSize,
+            structs::kPresContext_DefaultFixedFont_ID => self.mDefaultFixedSize,
+            structs::kGenericFont_serif => self.mDefaultSerifSize,
+            structs::kGenericFont_sans_serif => self.mDefaultSansSerifSize,
+            structs::kGenericFont_monospace => self.mDefaultMonospaceSize,
+            structs::kGenericFont_cursive => self.mDefaultCursiveSize,
+            structs::kGenericFont_fantasy => self.mDefaultFantasySize,
+            x => unreachable!("Unknown generic ID {}", x),
+        })
+    }
+}
+
 impl<'le> TElement for GeckoElement<'le> {
     type ConcreteNode = GeckoNode<'le>;
+    type FontMetricsProvider = GeckoFontMetricsProvider;
 
     fn as_node(&self) -> Self::ConcreteNode {
         unsafe { GeckoNode(&*(self.0 as *const _ as *const RawGeckoNode)) }
diff --git a/components/style/gecko_bindings/bindings.rs b/components/style/gecko_bindings/bindings.rs
index 2420617ab34..ba6f97defcf 100644
--- a/components/style/gecko_bindings/bindings.rs
+++ b/components/style/gecko_bindings/bindings.rs
@@ -27,6 +27,7 @@ use gecko_bindings::structs::TraversalRootBehavior;
 use gecko_bindings::structs::ComputedTimingFunction_BeforeFlag;
 use gecko_bindings::structs::FontFamilyList;
 use gecko_bindings::structs::FontFamilyType;
+use gecko_bindings::structs::FontSizePrefs;
 use gecko_bindings::structs::Keyframe;
 use gecko_bindings::structs::ServoBundledURI;
 use gecko_bindings::structs::ServoElementSnapshot;
@@ -1062,10 +1063,7 @@ extern "C" {
                                           aSource: *const nsStyleFont);
 }
 extern "C" {
-    pub fn Gecko_nsStyleFont_GetBaseSize(font: *const nsStyleFont,
-                                         pres_context:
-                                             RawGeckoPresContextBorrowed)
-     -> nscoord;
+    pub fn Gecko_GetBaseSize(lang: *mut nsIAtom) -> FontSizePrefs;
 }
 extern "C" {
     pub fn Gecko_GetMediaFeatures() -> *const nsMediaFeature;
diff --git a/components/style/gecko_bindings/structs_debug.rs b/components/style/gecko_bindings/structs_debug.rs
index 65a0dc3adf8..1c65c51d414 100644
--- a/components/style/gecko_bindings/structs_debug.rs
+++ b/components/style/gecko_bindings/structs_debug.rs
@@ -19547,6 +19547,8 @@ pub mod root {
     #[repr(C)]
     #[derive(Debug, Copy, Clone)]
     pub struct gfxMissingFontRecorder([u8; 0]);
+    pub const kPresContext_DefaultVariableFont_ID: u8 = 0;
+    pub const kPresContext_DefaultFixedFont_ID: u8 = 1;
     #[repr(C)]
     #[derive(Debug)]
     pub struct nsRootPresContext {
@@ -27150,6 +27152,67 @@ pub mod root {
         fn clone(&self) -> Self { *self }
     }
     #[repr(C)]
+    #[derive(Debug, Copy)]
+    pub struct FontSizePrefs {
+        pub mDefaultVariableSize: root::nscoord,
+        pub mDefaultFixedSize: root::nscoord,
+        pub mDefaultSerifSize: root::nscoord,
+        pub mDefaultSansSerifSize: root::nscoord,
+        pub mDefaultMonospaceSize: root::nscoord,
+        pub mDefaultCursiveSize: root::nscoord,
+        pub mDefaultFantasySize: root::nscoord,
+    }
+    #[test]
+    fn bindgen_test_layout_FontSizePrefs() {
+        assert_eq!(::std::mem::size_of::<FontSizePrefs>() , 28usize , concat !
+                   ( "Size of: " , stringify ! ( FontSizePrefs ) ));
+        assert_eq! (::std::mem::align_of::<FontSizePrefs>() , 4usize , concat
+                    ! ( "Alignment of " , stringify ! ( FontSizePrefs ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultVariableSize as * const _ as usize } , 0usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultVariableSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) . mDefaultFixedSize
+                    as * const _ as usize } , 4usize , concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultFixedSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) . mDefaultSerifSize
+                    as * const _ as usize } , 8usize , concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultSerifSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultSansSerifSize as * const _ as usize } , 12usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultSansSerifSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultMonospaceSize as * const _ as usize } , 16usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultMonospaceSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultCursiveSize as * const _ as usize } , 20usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultCursiveSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultFantasySize as * const _ as usize } , 24usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultFantasySize ) ));
+    }
+    impl Clone for FontSizePrefs {
+        fn clone(&self) -> Self { *self }
+    }
+    #[repr(C)]
     #[derive(Debug, Copy, Clone)]
     pub struct nsROCSSPrimitiveValue([u8; 0]);
     #[repr(C)]
diff --git a/components/style/gecko_bindings/structs_release.rs b/components/style/gecko_bindings/structs_release.rs
index 9d00e454f34..e75474a2b43 100644
--- a/components/style/gecko_bindings/structs_release.rs
+++ b/components/style/gecko_bindings/structs_release.rs
@@ -18981,6 +18981,8 @@ pub mod root {
     #[repr(C)]
     #[derive(Debug, Copy, Clone)]
     pub struct gfxMissingFontRecorder([u8; 0]);
+    pub const kPresContext_DefaultVariableFont_ID: u8 = 0;
+    pub const kPresContext_DefaultFixedFont_ID: u8 = 1;
     #[repr(C)]
     #[derive(Debug)]
     pub struct nsRootPresContext {
@@ -26491,6 +26493,67 @@ pub mod root {
         fn clone(&self) -> Self { *self }
     }
     #[repr(C)]
+    #[derive(Debug, Copy)]
+    pub struct FontSizePrefs {
+        pub mDefaultVariableSize: root::nscoord,
+        pub mDefaultFixedSize: root::nscoord,
+        pub mDefaultSerifSize: root::nscoord,
+        pub mDefaultSansSerifSize: root::nscoord,
+        pub mDefaultMonospaceSize: root::nscoord,
+        pub mDefaultCursiveSize: root::nscoord,
+        pub mDefaultFantasySize: root::nscoord,
+    }
+    #[test]
+    fn bindgen_test_layout_FontSizePrefs() {
+        assert_eq!(::std::mem::size_of::<FontSizePrefs>() , 28usize , concat !
+                   ( "Size of: " , stringify ! ( FontSizePrefs ) ));
+        assert_eq! (::std::mem::align_of::<FontSizePrefs>() , 4usize , concat
+                    ! ( "Alignment of " , stringify ! ( FontSizePrefs ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultVariableSize as * const _ as usize } , 0usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultVariableSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) . mDefaultFixedSize
+                    as * const _ as usize } , 4usize , concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultFixedSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) . mDefaultSerifSize
+                    as * const _ as usize } , 8usize , concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultSerifSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultSansSerifSize as * const _ as usize } , 12usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultSansSerifSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultMonospaceSize as * const _ as usize } , 16usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultMonospaceSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultCursiveSize as * const _ as usize } , 20usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultCursiveSize ) ));
+        assert_eq! (unsafe {
+                    & ( * ( 0 as * const FontSizePrefs ) ) .
+                    mDefaultFantasySize as * const _ as usize } , 24usize ,
+                    concat ! (
+                    "Alignment of field: " , stringify ! ( FontSizePrefs ) ,
+                    "::" , stringify ! ( mDefaultFantasySize ) ));
+    }
+    impl Clone for FontSizePrefs {
+        fn clone(&self) -> Self { *self }
+    }
+    #[repr(C)]
     #[derive(Debug, Copy, Clone)]
     pub struct nsROCSSPrimitiveValue([u8; 0]);
     #[repr(C)]
diff --git a/components/style/gecko_string_cache/mod.rs b/components/style/gecko_string_cache/mod.rs
index b10fd58174a..ca2c37ec213 100644
--- a/components/style/gecko_string_cache/mod.rs
+++ b/components/style/gecko_string_cache/mod.rs
@@ -186,10 +186,11 @@ impl fmt::Display for WeakAtom {
 
 impl Atom {
     /// Execute a callback with the atom represented by `ptr`.
-    pub unsafe fn with<F>(ptr: *mut nsIAtom, callback: &mut F) where F: FnMut(&Atom) {
+    pub unsafe fn with<F, R: 'static>(ptr: *mut nsIAtom, callback: &mut F) -> R where F: FnMut(&Atom) -> R {
         let atom = Atom(WeakAtom::new(ptr));
-        callback(&atom);
+        let ret = callback(&atom);
         mem::forget(atom);
+        ret
     }
 
     /// Creates an atom from an static atom pointer without checking in release
diff --git a/components/style/matching.rs b/components/style/matching.rs
index f0f288ef00f..7fab97d7d7a 100644
--- a/components/style/matching.rs
+++ b/components/style/matching.rs
@@ -15,6 +15,7 @@ use cascade_info::CascadeInfo;
 use context::{SequentialTask, SharedStyleContext, StyleContext};
 use data::{ComputedStyle, ElementData, ElementStyles, RestyleData};
 use dom::{AnimationRules, SendElement, TElement, TNode};
+use font_metrics::FontMetricsProvider;
 use properties::{CascadeFlags, ComputedValues, SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP, cascade};
 use properties::longhands::display::computed_value as display;
 use restyle_hints::{RESTYLE_STYLE_ATTRIBUTE, RESTYLE_CSS_ANIMATIONS, RestyleHint};
@@ -323,6 +324,7 @@ trait PrivateMatchMethods: TElement {
 
     fn cascade_with_rules(&self,
                           shared_context: &SharedStyleContext,
+                          font_metrics_provider: &FontMetricsProvider,
                           rule_node: &StrongRuleNode,
                           primary_style: &ComputedStyle,
                           cascade_flags: CascadeFlags,
@@ -388,6 +390,7 @@ trait PrivateMatchMethods: TElement {
                              layout_parent_style,
                              Some(&mut cascade_info),
                              &*shared_context.error_reporter,
+                             font_metrics_provider,
                              cascade_flags));
 
         cascade_info.finish(&self.as_node());
@@ -406,7 +409,8 @@ trait PrivateMatchMethods: TElement {
 
         // Grab the rule node.
         let rule_node = &pseudo_style.as_ref().map_or(primary_style, |p| &*p.1).rules;
-        self.cascade_with_rules(context.shared, rule_node, primary_style, cascade_flags, pseudo_style.is_some())
+        self.cascade_with_rules(context.shared, &context.thread_local.font_metrics_provider,
+                                rule_node, primary_style, cascade_flags, pseudo_style.is_some())
     }
 
     /// Computes values and damage for the primary or pseudo style of an element,
@@ -470,6 +474,7 @@ trait PrivateMatchMethods: TElement {
             cascade_flags.insert(SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP)
         }
         self.cascade_with_rules(context.shared,
+                                &context.thread_local.font_metrics_provider,
                                 &without_transition_rules,
                                 primary_style,
                                 cascade_flags,
@@ -531,7 +536,8 @@ trait PrivateMatchMethods: TElement {
         let shared_context = context.shared;
         if let Some(ref mut old) = *old_values {
             self.update_animations_for_cascade(shared_context, old,
-                                               possibly_expired_animations);
+                                               possibly_expired_animations,
+                                               &context.thread_local.font_metrics_provider);
         }
 
         let new_animations_sender = &context.thread_local.new_animations_sender;
@@ -606,7 +612,8 @@ trait PrivateMatchMethods: TElement {
     fn update_animations_for_cascade(&self,
                                      context: &SharedStyleContext,
                                      style: &mut Arc<ComputedValues>,
-                                     possibly_expired_animations: &mut Vec<PropertyAnimation>) {
+                                     possibly_expired_animations: &mut Vec<PropertyAnimation>,
+                                     font_metrics: &FontMetricsProvider) {
         // Finish any expired transitions.
         let this_opaque = self.as_node().opaque();
         animation::complete_expired_transitions(this_opaque, style, context);
@@ -633,7 +640,8 @@ trait PrivateMatchMethods: TElement {
                 if !running_animation.is_expired() {
                     animation::update_style_for_animation(context,
                                                           running_animation,
-                                                          style);
+                                                          style,
+                                                          font_metrics);
                     if let Animation::Transition(_, _, _, ref frame, _) = *running_animation {
                         possibly_expired_animations.push(frame.property_animation.clone())
                     }
@@ -1145,6 +1153,7 @@ pub trait MatchMethods : TElement {
     /// Returns computed values without animation and transition rules.
     fn get_base_style(&self,
                       shared_context: &SharedStyleContext,
+                      font_metrics_provider: &FontMetricsProvider,
                       primary_style: &ComputedStyle,
                       pseudo_style: &Option<(&PseudoElement, &ComputedStyle)>)
                       -> Arc<ComputedValues> {
@@ -1163,6 +1172,7 @@ pub trait MatchMethods : TElement {
             cascade_flags.insert(SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP)
         }
         self.cascade_with_rules(shared_context,
+                                font_metrics_provider,
                                 &without_animation_rules,
                                 primary_style,
                                 cascade_flags,
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index b7ef293239c..64c83d418c5 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -536,7 +536,8 @@ ${helpers.single_keyword("font-variant-caps",
             type ComputedValue = Au;
             #[inline]
             fn to_computed_value(&self, cx: &Context) -> computed_value::T {
-                use gecko_bindings::bindings::Gecko_nsStyleFont_GetBaseSize;
+                use gecko_bindings::bindings::Gecko_GetBaseSize;
+                use gecko_bindings::structs;
                 use values::specified::length::au_to_int_px;
                 // Data from nsRuleNode.cpp in Gecko
                 // Mapping from base size and HTML size to pixels
@@ -561,10 +562,11 @@ ${helpers.single_keyword("font-variant-caps",
 
                 // XXXManishearth handle quirks mode
 
-                let base_size = unsafe {
-                    Gecko_nsStyleFont_GetBaseSize(cx.style().get_font().gecko(),
-                                                  &*cx.device.pres_context)
-                };
+                let ref gecko_font = cx.style().get_font().gecko();
+                let base_size = unsafe { Atom::with(gecko_font.mLanguage.raw(), &mut |atom| {
+                    cx.font_metrics_provider.get_size(atom, gecko_font.mGenericID).0
+                }) };
+
                 let base_size_px = au_to_int_px(base_size as f32);
                 let html_size = *self as usize;
                 if base_size_px >= 9 && base_size_px <= 16 {
diff --git a/components/style/properties/properties.mako.rs b/components/style/properties/properties.mako.rs
index 88121aa32f3..d1c8e0849af 100644
--- a/components/style/properties/properties.mako.rs
+++ b/components/style/properties/properties.mako.rs
@@ -1944,6 +1944,7 @@ pub fn cascade(device: &Device,
                layout_parent_style: Option<<&ComputedValues>,
                cascade_info: Option<<&mut CascadeInfo>,
                error_reporter: &ParseErrorReporter,
+               font_metrics_provider: &FontMetricsProvider,
                flags: CascadeFlags)
                -> ComputedValues {
     debug_assert_eq!(parent_style.is_some(), layout_parent_style.is_some());
@@ -1988,7 +1989,7 @@ pub fn cascade(device: &Device,
                        layout_parent_style,
                        cascade_info,
                        error_reporter,
-                       None,
+                       font_metrics_provider,
                        flags)
 }
 
@@ -2002,7 +2003,7 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
                                     layout_parent_style: &ComputedValues,
                                     mut cascade_info: Option<<&mut CascadeInfo>,
                                     error_reporter: &ParseErrorReporter,
-                                    font_metrics_provider: Option<<&FontMetricsProvider>,
+                                    font_metrics_provider: &FontMetricsProvider,
                                     flags: CascadeFlags)
                                     -> ComputedValues
     where F: Fn() -> I,
@@ -2109,6 +2110,7 @@ pub fn apply_declarations<'a, F, I>(device: &Device,
                     | LonghandId::TextOrientation
                     | LonghandId::AnimationName
                     | LonghandId::TransitionProperty
+                    | LonghandId::XLang
                 % endif
             );
             if
diff --git a/components/style/servo/media_queries.rs b/components/style/servo/media_queries.rs
index 75bd789766f..95a537bfdfb 100644
--- a/components/style/servo/media_queries.rs
+++ b/components/style/servo/media_queries.rs
@@ -7,6 +7,7 @@
 use app_units::Au;
 use cssparser::Parser;
 use euclid::{Size2D, TypedSize2D};
+use font_metrics::ServoMetricsProvider;
 use media_queries::MediaType;
 use properties::ComputedValues;
 use std::fmt;
@@ -185,7 +186,10 @@ impl Range<specified::Length> {
             // This cloning business is kind of dumb.... It's because Context
             // insists on having an actual ComputedValues inside itself.
             style: default_values.clone(),
-            font_metrics_provider: None
+            // Servo doesn't support font metrics
+            // A real provider will be needed here once we do; since
+            // ch units can exist in media queries.
+            font_metrics_provider: &ServoMetricsProvider,
         };
 
         match *self {
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 2982457364a..4731ec84cb9 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -10,6 +10,7 @@ use {Atom, LocalName};
 use data::ComputedStyle;
 use dom::{AnimationRules, PresentationalHintsSynthetizer, TElement};
 use error_reporting::StdoutErrorReporter;
+use font_metrics::FontMetricsProvider;
 use keyframes::KeyframesAnimation;
 use media_queries::Device;
 use pdqsort::sort_by;
@@ -345,7 +346,8 @@ impl Stylist {
                                          guards: &StylesheetGuards,
                                          pseudo: &PseudoElement,
                                          parent: Option<&Arc<ComputedValues>>,
-                                         cascade_flags: CascadeFlags)
+                                         cascade_flags: CascadeFlags,
+                                         font_metrics: &FontMetricsProvider)
                                          -> ComputedStyle {
         debug_assert!(pseudo.is_precomputed());
 
@@ -381,6 +383,7 @@ impl Stylist {
                                 parent.map(|p| &**p),
                                 None,
                                 &StdoutErrorReporter,
+                                font_metrics,
                                 cascade_flags);
         ComputedStyle::new(rule_node, Arc::new(computed))
     }
@@ -392,6 +395,8 @@ impl Stylist {
                                pseudo: &PseudoElement,
                                parent_style: &Arc<ComputedValues>)
                                -> Arc<ComputedValues> {
+        use font_metrics::ServoMetricsProvider;
+
         // For most (but not all) pseudo-elements, we inherit all values from the parent.
         let inherit_all = match *pseudo {
             PseudoElement::ServoText |
@@ -416,7 +421,8 @@ impl Stylist {
         if inherit_all {
             cascade_flags.insert(INHERIT_ALL);
         }
-        self.precomputed_values_for_pseudo(guards, &pseudo, Some(parent_style), cascade_flags)
+        self.precomputed_values_for_pseudo(guards, &pseudo, Some(parent_style), cascade_flags,
+                                           &ServoMetricsProvider)
             .values.unwrap()
     }
 
@@ -431,7 +437,8 @@ impl Stylist {
                                                   guards: &StylesheetGuards,
                                                   element: &E,
                                                   pseudo: &PseudoElement,
-                                                  parent: &Arc<ComputedValues>)
+                                                  parent: &Arc<ComputedValues>,
+                                                  font_metrics: &FontMetricsProvider)
                                                   -> Option<ComputedStyle>
         where E: TElement +
                  fmt::Debug +
@@ -494,6 +501,7 @@ impl Stylist {
                                 Some(&**parent),
                                 None,
                                 &StdoutErrorReporter,
+                                font_metrics,
                                 CascadeFlags::empty());
 
         Some(ComputedStyle::new(rule_node, Arc::new(computed)))
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index b2b64e1c212..47d809c5ebb 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -57,9 +57,7 @@ pub struct Context<'a> {
 
     /// A font metrics provider, used to access font metrics to implement
     /// font-relative units.
-    ///
-    /// TODO(emilio): This should be required, see #14079.
-    pub font_metrics_provider: Option<&'a FontMetricsProvider>,
+    pub font_metrics_provider: &'a FontMetricsProvider,
 }
 
 impl<'a> Context<'a> {
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 5e96e0e40f6..d6dcad04a3b 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -73,11 +73,9 @@ impl FontRelativeLength {
     /// font-family list.
     pub fn find_first_available_font_metrics(context: &Context) -> Option<FontMetrics> {
         use font_metrics::FontMetricsQueryResult::*;
-        if let Some(ref metrics_provider) = context.font_metrics_provider {
-            for family in context.style().get_font().font_family_iter() {
-                if let Available(metrics) = metrics_provider.query(family.atom()) {
-                    return metrics;
-                }
+        for family in context.style().get_font().font_family_iter() {
+            if let Available(metrics) = context.font_metrics_provider.query(family.atom()) {
+                return metrics;
             }
         }
 
diff --git a/components/style/viewport.rs b/components/style/viewport.rs
index d2d2af77480..22426a2fbc6 100644
--- a/components/style/viewport.rs
+++ b/components/style/viewport.rs
@@ -13,6 +13,7 @@ use app_units::Au;
 use cssparser::{AtRuleParser, DeclarationListParser, DeclarationParser, Parser, parse_important};
 use cssparser::ToCss as ParserToCss;
 use euclid::size::TypedSize2D;
+use font_metrics::get_metrics_provider_for_product;
 use media_queries::Device;
 use parser::{Parse, ParserContext, log_css_error};
 use shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
@@ -672,6 +673,8 @@ impl MaybeNew for ViewportConstraints {
         // resolved against initial values
         let initial_viewport = device.au_viewport_size();
 
+        let provider = get_metrics_provider_for_product();
+
         // TODO(emilio): Stop cloning `ComputedValues` around!
         let context = Context {
             is_root_element: false,
@@ -679,7 +682,7 @@ impl MaybeNew for ViewportConstraints {
             inherited_style: device.default_computed_values(),
             layout_parent_style: device.default_computed_values(),
             style: device.default_computed_values().clone(),
-            font_metrics_provider: None, // TODO: Should have!
+            font_metrics_provider: &provider,
         };
 
         // DEVICE-ADAPT § 9.3 Resolving 'extend-to-zoom'
diff --git a/ports/geckolib/glue.rs b/ports/geckolib/glue.rs
index 99d48fe58e5..2e65da44ab0 100644
--- a/ports/geckolib/glue.rs
+++ b/ports/geckolib/glue.rs
@@ -20,6 +20,7 @@ use style::data::{ElementData, ElementStyles, RestyleData};
 use style::dom::{AnimationOnlyDirtyDescendants, DirtyDescendants};
 use style::dom::{ShowSubtreeData, TElement, TNode};
 use style::error_reporting::StdoutErrorReporter;
+use style::font_metrics::get_metrics_provider_for_product;
 use style::gecko::data::{PerDocumentStyleData, PerDocumentStyleDataImpl};
 use style::gecko::global_style_data::GLOBAL_STYLE_DATA;
 use style::gecko::restyle_damage::GeckoRestyleDamage;
@@ -418,7 +419,8 @@ pub extern "C" fn Servo_StyleSet_GetBaseComputedValuesForElement(raw_data: RawSe
     let pseudos = &styles.pseudos;
     let pseudo_style = pseudo.as_ref().map(|p| (p, pseudos.get(p).unwrap()));
 
-    element.get_base_style(shared_context, &styles.primary, &pseudo_style)
+    let provider = get_metrics_provider_for_product();
+    element.get_base_style(shared_context, &provider, &styles.primary, &pseudo_style)
            .into_strong()
 }
 
@@ -812,8 +814,9 @@ pub extern "C" fn Servo_ComputedValues_GetForAnonymousBox(parent_style_or_null:
     if skip_display_fixup {
         cascade_flags.insert(SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP);
     }
+    let metrics = get_metrics_provider_for_product();
     data.stylist.precomputed_values_for_pseudo(&guards, &pseudo, maybe_parent,
-                                               cascade_flags)
+                                               cascade_flags, &metrics)
         .values.unwrap()
         .into_strong()
 }
@@ -859,10 +862,12 @@ fn get_pseudo_style(guard: &SharedRwLockReadGuard, element: GeckoElement, pseudo
             let d = doc_data.borrow_mut();
             let base = styles.primary.values();
             let guards = StylesheetGuards::same(guard);
+            let metrics = get_metrics_provider_for_product();
             d.stylist.lazily_compute_pseudo_element_style(&guards,
                                                           &element,
                                                           &pseudo,
-                                                          base)
+                                                          base,
+                                                          &metrics)
                      .map(|s| s.values().clone())
         },
     }
@@ -1672,6 +1677,7 @@ pub extern "C" fn Servo_GetComputedKeyframeValues(keyframes: RawGeckoKeyframeLis
     let parent_style = parent_style.as_ref().map(|r| &**ComputedValues::as_arc(&r));
 
     let default_values = data.default_computed_values();
+    let metrics = get_metrics_provider_for_product();
 
     let context = Context {
         is_root_element: false,
@@ -1679,7 +1685,7 @@ pub extern "C" fn Servo_GetComputedKeyframeValues(keyframes: RawGeckoKeyframeLis
         inherited_style: parent_style.unwrap_or(default_values),
         layout_parent_style: parent_style.unwrap_or(default_values),
         style: (**style).clone(),
-        font_metrics_provider: None,
+        font_metrics_provider: &metrics,
     };
 
     for (index, keyframe) in keyframes.iter().enumerate() {
diff --git a/tests/unit/style/parsing/image.rs b/tests/unit/style/parsing/image.rs
index 7b42def9758..3e3bce84f89 100644
--- a/tests/unit/style/parsing/image.rs
+++ b/tests/unit/style/parsing/image.rs
@@ -6,6 +6,7 @@ use cssparser::Parser;
 use euclid::size::TypedSize2D;
 use media_queries::CSSErrorReporterTest;
 use std::f32::consts::PI;
+use style::font_metrics::ServoMetricsProvider;
 use style::media_queries::{Device, MediaType};
 use style::parser::ParserContext;
 use style::properties::ComputedValues;
@@ -51,7 +52,7 @@ fn test_linear_gradient() {
         inherited_style: initial_style,
         layout_parent_style: initial_style,
         style: initial_style.clone(),
-        font_metrics_provider: None,
+        font_metrics_provider: &ServoMetricsProvider,
     };
     assert_eq!(specified::AngleOrCorner::None.to_computed_value(&specified_context),
                computed::AngleOrCorner::Angle(Angle::from_radians(PI)));