Reuse StylesheetContent for inline style sheets with identical content (#38540)

For duplicate style sheets with identical content, `StylesheetContents`
can be reused to avoid redundant parsing of the inline style sheets.
Since duplicate stylesheets is a common case with web components, this
change will significantly improve performance. Additionally, the cache
hit rate of stylo's `CascadeDataCache` can now be significantly
improved.

When shared `StylesheetContents` is modified, copy-on-write will occur
to avoid affecting other sharers. And then updates the references to
`CssRule` or `PropertyDeclarationBlock` stored in the CSSOMs to ensure
that modifications are made only on the new copy.

Signed-off-by: sharpshooter_pt <ibluegalaxy_taoj@163.com>

components/script/dom/cssconditionrule.rs

@@ -2,9 +2,11 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use servo_arc::Arc;
-use style::shared_lock::{Locked, SharedRwLock};
+use style::shared_lock::{Locked, SharedRwLock, SharedRwLockReadGuard};
 use style::stylesheets::CssRules as StyleCssRules;
 
 use crate::dom::bindings::codegen::Bindings::CSSConditionRuleBinding::CSSConditionRuleMethods;
@@ -20,7 +22,7 @@ pub(crate) struct CSSConditionRule {
     cssgroupingrule: CSSGroupingRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    rules: Arc<Locked<StyleCssRules>>,
+    rules: RefCell<Arc<Locked<StyleCssRules>>>,
 }
 
 impl CSSConditionRule {
@@ -30,7 +32,7 @@ impl CSSConditionRule {
     ) -> CSSConditionRule {
         CSSConditionRule {
             cssgroupingrule: CSSGroupingRule::new_inherited(parent_stylesheet),
-            rules,
+            rules: RefCell::new(rules),
         }
     }
 
@@ -43,7 +45,16 @@ impl CSSConditionRule {
     }
 
     pub(crate) fn clone_rules(&self) -> Arc<Locked<StyleCssRules>> {
-        self.rules.clone()
+        self.rules.borrow().clone()
+    }
+
+    pub(crate) fn update_rules(
+        &self,
+        rules: Arc<Locked<StyleCssRules>>,
+        guard: &SharedRwLockReadGuard,
+    ) {
+        self.cssgroupingrule.update_rules(&rules, guard);
+        *self.rules.borrow_mut() = rules;
     }
 }
 

components/script/dom/cssfontfacerule.rs

@@ -2,6 +2,8 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use servo_arc::Arc;
 use style::shared_lock::{Locked, ToCssWithGuard};
@@ -20,7 +22,7 @@ pub(crate) struct CSSFontFaceRule {
     cssrule: CSSRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    fontfacerule: Arc<Locked<FontFaceRule>>,
+    fontfacerule: RefCell<Arc<Locked<FontFaceRule>>>,
 }
 
 impl CSSFontFaceRule {
@@ -30,7 +32,7 @@ impl CSSFontFaceRule {
     ) -> CSSFontFaceRule {
         CSSFontFaceRule {
             cssrule: CSSRule::new_inherited(parent_stylesheet),
-            fontfacerule,
+            fontfacerule: RefCell::new(fontfacerule),
         }
     }
 
@@ -50,6 +52,10 @@ impl CSSFontFaceRule {
             can_gc,
         )
     }
+
+    pub(crate) fn update_rule(&self, fontfacerule: Arc<Locked<FontFaceRule>>) {
+        *self.fontfacerule.borrow_mut() = fontfacerule;
+    }
 }
 
 impl SpecificCSSRule for CSSFontFaceRule {
@@ -60,6 +66,7 @@ impl SpecificCSSRule for CSSFontFaceRule {
     fn get_css(&self) -> DOMString {
         let guard = self.cssrule.shared_lock().read();
         self.fontfacerule
+            .borrow()
             .read_with(&guard)
             .to_css_string(&guard)
             .into()

components/script/dom/cssgroupingrule.rs

@@ -3,8 +3,9 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use dom_struct::dom_struct;
-use style::shared_lock::SharedRwLock;
+use servo_arc::Arc;
-use style::stylesheets::{CssRuleType, CssRuleTypes};
+use style::shared_lock::{Locked, SharedRwLock, SharedRwLockReadGuard};
+use style::stylesheets::{CssRuleType, CssRuleTypes, CssRules};
 
 use crate::dom::bindings::codegen::Bindings::CSSGroupingRuleBinding::CSSGroupingRuleMethods;
 use crate::dom::bindings::error::{ErrorResult, Fallible};
@@ -62,6 +63,16 @@ impl CSSGroupingRule {
     pub(crate) fn shared_lock(&self) -> &SharedRwLock {
         self.cssrule.shared_lock()
     }
+
+    pub(crate) fn update_rules(
+        &self,
+        rules: &Arc<Locked<CssRules>>,
+        guard: &SharedRwLockReadGuard,
+    ) {
+        if let Some(rulelist) = self.rulelist.get() {
+            rulelist.update_rules(RulesSource::Rules(rules.clone()), guard);
+        }
+    }
 }
 
 impl CSSGroupingRuleMethods<crate::DomTypeHolder> for CSSGroupingRule {

components/script/dom/cssimportrule.rs

@@ -2,6 +2,8 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use servo_arc::Arc;
 use style::shared_lock::{Locked, ToCssWithGuard};
@@ -23,7 +25,7 @@ pub(crate) struct CSSImportRule {
     cssrule: CSSRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    import_rule: Arc<Locked<ImportRule>>,
+    import_rule: RefCell<Arc<Locked<ImportRule>>>,
 }
 
 impl CSSImportRule {
@@ -33,7 +35,7 @@ impl CSSImportRule {
     ) -> Self {
         CSSImportRule {
             cssrule: CSSRule::new_inherited(parent_stylesheet),
-            import_rule,
+            import_rule: RefCell::new(import_rule),
         }
     }
 
@@ -50,6 +52,10 @@ impl CSSImportRule {
             can_gc,
         )
     }
+
+    pub(crate) fn update_rule(&self, import_rule: Arc<Locked<ImportRule>>) {
+        *self.import_rule.borrow_mut() = import_rule;
+    }
 }
 
 impl SpecificCSSRule for CSSImportRule {
@@ -60,6 +66,7 @@ impl SpecificCSSRule for CSSImportRule {
     fn get_css(&self) -> DOMString {
         let guard = self.cssrule.shared_lock().read();
         self.import_rule
+            .borrow()
             .read_with(&guard)
             .to_css_string(&guard)
             .into()
@@ -70,7 +77,7 @@ impl CSSImportRuleMethods<crate::DomTypeHolder> for CSSImportRule {
     /// <https://drafts.csswg.org/cssom-1/#dom-cssimportrule-layername>
     fn GetLayerName(&self) -> Option<DOMString> {
         let guard = self.cssrule.shared_lock().read();
-        match &self.import_rule.read_with(&guard).layer {
+        match &self.import_rule.borrow().read_with(&guard).layer {
             ImportLayer::None => None,
             ImportLayer::Anonymous => Some(DOMString::new()),
             ImportLayer::Named(name) => Some(DOMString::from_string(name.to_css_string())),

components/script/dom/csskeyframerule.rs

@@ -2,9 +2,11 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use servo_arc::Arc;
-use style::shared_lock::{Locked, ToCssWithGuard};
+use style::shared_lock::{Locked, SharedRwLockReadGuard, ToCssWithGuard};
 use style::stylesheets::CssRuleType;
 use style::stylesheets::keyframes_rule::Keyframe;
 
@@ -24,7 +26,7 @@ pub(crate) struct CSSKeyframeRule {
     cssrule: CSSRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    keyframerule: Arc<Locked<Keyframe>>,
+    keyframerule: RefCell<Arc<Locked<Keyframe>>>,
     style_decl: MutNullableDom<CSSStyleDeclaration>,
 }
 
@@ -35,7 +37,7 @@ impl CSSKeyframeRule {
     ) -> CSSKeyframeRule {
         CSSKeyframeRule {
             cssrule: CSSRule::new_inherited(parent_stylesheet),
-            keyframerule,
+            keyframerule: RefCell::new(keyframerule),
             style_decl: Default::default(),
         }
     }
@@ -56,6 +58,17 @@ impl CSSKeyframeRule {
             can_gc,
         )
     }
+
+    pub(crate) fn update_rule(
+        &self,
+        keyframerule: Arc<Locked<Keyframe>>,
+        guard: &SharedRwLockReadGuard,
+    ) {
+        if let Some(ref style_decl) = self.style_decl.get() {
+            style_decl.update_property_declaration_block(&keyframerule.read_with(guard).block);
+        }
+        *self.keyframerule.borrow_mut() = keyframerule;
+    }
 }
 
 impl CSSKeyframeRuleMethods<crate::DomTypeHolder> for CSSKeyframeRule {
@@ -67,7 +80,7 @@ impl CSSKeyframeRuleMethods<crate::DomTypeHolder> for CSSKeyframeRule {
                 self.global().as_window(),
                 CSSStyleOwner::CSSRule(
                     Dom::from_ref(self.upcast()),
-                    self.keyframerule.read_with(&guard).block.clone(),
+                    RefCell::new(self.keyframerule.borrow().read_with(&guard).block.clone()),
                 ),
                 None,
                 CSSModificationAccess::ReadWrite,
@@ -85,6 +98,7 @@ impl SpecificCSSRule for CSSKeyframeRule {
     fn get_css(&self) -> DOMString {
         let guard = self.cssrule.shared_lock().read();
         self.keyframerule
+            .borrow()
             .read_with(&guard)
             .to_css_string(&guard)
             .into()

components/script/dom/csskeyframesrule.rs

@@ -2,10 +2,12 @@
  * 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 std::cell::RefCell;
+
 use cssparser::{Parser, ParserInput};
 use dom_struct::dom_struct;
 use servo_arc::Arc;
-use style::shared_lock::{Locked, ToCssWithGuard};
+use style::shared_lock::{Locked, SharedRwLockReadGuard, ToCssWithGuard};
 use style::stylesheets::CssRuleType;
 use style::stylesheets::keyframes_rule::{Keyframe, KeyframeSelector, KeyframesRule};
 use style::values::KeyframesName;
@@ -28,7 +30,7 @@ pub(crate) struct CSSKeyframesRule {
     cssrule: CSSRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    keyframesrule: Arc<Locked<KeyframesRule>>,
+    keyframesrule: RefCell<Arc<Locked<KeyframesRule>>>,
     rulelist: MutNullableDom<CSSRuleList>,
 }
 
@@ -39,7 +41,7 @@ impl CSSKeyframesRule {
     ) -> CSSKeyframesRule {
         CSSKeyframesRule {
             cssrule: CSSRule::new_inherited(parent_stylesheet),
-            keyframesrule,
+            keyframesrule: RefCell::new(keyframesrule),
             rulelist: MutNullableDom::new(None),
         }
     }
@@ -67,7 +69,7 @@ impl CSSKeyframesRule {
             CSSRuleList::new(
                 self.global().as_window(),
                 parent_stylesheet,
-                RulesSource::Keyframes(self.keyframesrule.clone()),
+                RulesSource::Keyframes(self.keyframesrule.borrow().clone()),
                 can_gc,
             )
         })
@@ -82,6 +84,7 @@ impl CSSKeyframesRule {
             // This finds the *last* element matching a selector
             // because that's the rule that applies. Thus, rposition
             self.keyframesrule
+                .borrow()
                 .read_with(&guard)
                 .keyframes
                 .iter()
@@ -90,6 +93,18 @@ impl CSSKeyframesRule {
             None
         }
     }
+
+    pub(crate) fn update_rule(
+        &self,
+        keyframesrule: Arc<Locked<KeyframesRule>>,
+        guard: &SharedRwLockReadGuard,
+    ) {
+        if let Some(rulelist) = self.rulelist.get() {
+            rulelist.update_rules(RulesSource::Keyframes(keyframesrule.clone()), guard);
+        }
+
+        *self.keyframesrule.borrow_mut() = keyframesrule;
+    }
 }
 
 impl CSSKeyframesRuleMethods<crate::DomTypeHolder> for CSSKeyframesRule {
@@ -108,8 +123,10 @@ impl CSSKeyframesRuleMethods<crate::DomTypeHolder> for CSSKeyframesRule {
         );
 
         if let Ok(rule) = rule {
+            self.cssrule.parent_stylesheet().will_modify();
             let mut guard = self.cssrule.shared_lock().write();
             self.keyframesrule
+                .borrow()
                 .write_with(&mut guard)
                 .keyframes
                 .push(rule);
@@ -135,7 +152,7 @@ impl CSSKeyframesRuleMethods<crate::DomTypeHolder> for CSSKeyframesRule {
     // https://drafts.csswg.org/css-animations/#dom-csskeyframesrule-name
     fn Name(&self) -> DOMString {
         let guard = self.cssrule.shared_lock().read();
-        DOMString::from(&**self.keyframesrule.read_with(&guard).name.as_atom())
+        DOMString::from(&**self.keyframesrule.borrow().read_with(&guard).name.as_atom())
     }
 
     // https://drafts.csswg.org/css-animations/#dom-csskeyframesrule-name
@@ -143,9 +160,10 @@ impl CSSKeyframesRuleMethods<crate::DomTypeHolder> for CSSKeyframesRule {
         // Spec deviation: https://github.com/w3c/csswg-drafts/issues/801
         // Setting this property to a CSS-wide keyword or `none` does not throw,
         // it stores a value that serializes as a quoted string.
+        self.cssrule.parent_stylesheet().will_modify();
         let name = KeyframesName::from_ident(&value);
         let mut guard = self.cssrule.shared_lock().write();
-        self.keyframesrule.write_with(&mut guard).name = name;
+        self.keyframesrule.borrow().write_with(&mut guard).name = name;
         self.cssrule.parent_stylesheet().notify_invalidations();
         Ok(())
     }
@@ -159,6 +177,7 @@ impl SpecificCSSRule for CSSKeyframesRule {
     fn get_css(&self) -> DOMString {
         let guard = self.cssrule.shared_lock().read();
         self.keyframesrule
+            .borrow()
             .read_with(&guard)
             .to_css_string(&guard)
             .into()

components/script/dom/csslayerblockrule.rs

@@ -2,9 +2,11 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use servo_arc::Arc;
-use style::shared_lock::{Locked, ToCssWithGuard};
+use style::shared_lock::{Locked, SharedRwLockReadGuard, ToCssWithGuard};
 use style::stylesheets::{CssRuleType, CssRules, LayerBlockRule};
 use style_traits::ToCss;
 
@@ -23,7 +25,7 @@ pub(crate) struct CSSLayerBlockRule {
     cssgroupingrule: CSSGroupingRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    layerblockrule: Arc<LayerBlockRule>,
+    layerblockrule: RefCell<Arc<LayerBlockRule>>,
 }
 
 impl CSSLayerBlockRule {
@@ -33,7 +35,7 @@ impl CSSLayerBlockRule {
     ) -> CSSLayerBlockRule {
         CSSLayerBlockRule {
             cssgroupingrule: CSSGroupingRule::new_inherited(parent_stylesheet),
-            layerblockrule,
+            layerblockrule: RefCell::new(layerblockrule),
         }
     }
 
@@ -55,7 +57,17 @@ impl CSSLayerBlockRule {
     }
 
     pub(crate) fn clone_rules(&self) -> Arc<Locked<CssRules>> {
-        self.layerblockrule.rules.clone()
+        self.layerblockrule.borrow().rules.clone()
+    }
+
+    pub(crate) fn update_rule(
+        &self,
+        layerblockrule: Arc<LayerBlockRule>,
+        guard: &SharedRwLockReadGuard,
+    ) {
+        self.cssgroupingrule
+            .update_rules(&layerblockrule.rules, guard);
+        *self.layerblockrule.borrow_mut() = layerblockrule;
     }
 }
 
@@ -66,14 +78,14 @@ impl SpecificCSSRule for CSSLayerBlockRule {
 
     fn get_css(&self) -> DOMString {
         let guard = self.cssgroupingrule.shared_lock().read();
-        self.layerblockrule.to_css_string(&guard).into()
+        self.layerblockrule.borrow().to_css_string(&guard).into()
     }
 }
 
 impl CSSLayerBlockRuleMethods<crate::DomTypeHolder> for CSSLayerBlockRule {
     /// <https://drafts.csswg.org/css-cascade-5/#dom-csslayerblockrule-name>
     fn Name(&self) -> DOMString {
-        if let Some(name) = &self.layerblockrule.name {
+        if let Some(name) = &self.layerblockrule.borrow().name {
             DOMString::from_string(name.to_css_string())
         } else {
             DOMString::new()

components/script/dom/csslayerstatementrule.rs

@@ -2,6 +2,8 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use js::rust::MutableHandleValue;
 use servo_arc::Arc;
@@ -24,7 +26,7 @@ pub(crate) struct CSSLayerStatementRule {
     cssrule: CSSRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    layerstatementrule: Arc<LayerStatementRule>,
+    layerstatementrule: RefCell<Arc<LayerStatementRule>>,
 }
 
 impl CSSLayerStatementRule {
@@ -34,7 +36,7 @@ impl CSSLayerStatementRule {
     ) -> CSSLayerStatementRule {
         CSSLayerStatementRule {
             cssrule: CSSRule::new_inherited(parent_stylesheet),
-            layerstatementrule,
+            layerstatementrule: RefCell::new(layerstatementrule),
         }
     }
 
@@ -54,6 +56,10 @@ impl CSSLayerStatementRule {
             can_gc,
         )
     }
+
+    pub(crate) fn update_rule(&self, layerstatementrule: Arc<LayerStatementRule>) {
+        *self.layerstatementrule.borrow_mut() = layerstatementrule;
+    }
 }
 
 impl SpecificCSSRule for CSSLayerStatementRule {
@@ -63,7 +69,10 @@ impl SpecificCSSRule for CSSLayerStatementRule {
 
     fn get_css(&self) -> DOMString {
         let guard = self.cssrule.shared_lock().read();
-        self.layerstatementrule.to_css_string(&guard).into()
+        self.layerstatementrule
+            .borrow()
+            .to_css_string(&guard)
+            .into()
     }
 }
 
@@ -72,6 +81,7 @@ impl CSSLayerStatementRuleMethods<crate::DomTypeHolder> for CSSLayerStatementRul
     fn NameList(&self, cx: SafeJSContext, can_gc: CanGc, retval: MutableHandleValue) {
         let names: Vec<DOMString> = self
             .layerstatementrule
+            .borrow()
             .names
             .iter()
             .map(|name| DOMString::from_string(name.to_css_string()))

components/script/dom/cssmediarule.rs

@@ -2,9 +2,11 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use servo_arc::Arc;
-use style::shared_lock::ToCssWithGuard;
+use style::shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
 use style::stylesheets::{CssRuleType, MediaRule};
 use style_traits::ToCss;
 
@@ -24,7 +26,7 @@ pub(crate) struct CSSMediaRule {
     cssconditionrule: CSSConditionRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    mediarule: Arc<MediaRule>,
+    mediarule: RefCell<Arc<MediaRule>>,
     medialist: MutNullableDom<MediaList>,
 }
 
@@ -33,7 +35,7 @@ impl CSSMediaRule {
         let list = mediarule.rules.clone();
         CSSMediaRule {
             cssconditionrule: CSSConditionRule::new_inherited(parent_stylesheet, list),
-            mediarule,
+            mediarule: RefCell::new(mediarule),
             medialist: MutNullableDom::new(None),
         }
     }
@@ -57,7 +59,7 @@ impl CSSMediaRule {
             MediaList::new(
                 self.global().as_window(),
                 self.cssconditionrule.parent_stylesheet(),
-                self.mediarule.media_queries.clone(),
+                self.mediarule.borrow().media_queries.clone(),
                 can_gc,
             )
         })
@@ -66,8 +68,21 @@ impl CSSMediaRule {
     /// <https://drafts.csswg.org/css-conditional-3/#the-cssmediarule-interface>
     pub(crate) fn get_condition_text(&self) -> DOMString {
         let guard = self.cssconditionrule.shared_lock().read();
-        let list = self.mediarule.media_queries.read_with(&guard);
+        self.mediarule
-        list.to_css_string().into()
+            .borrow()
+            .media_queries
+            .read_with(&guard)
+            .to_css_string()
+            .into()
+    }
+
+    pub(crate) fn update_rule(&self, mediarule: Arc<MediaRule>, guard: &SharedRwLockReadGuard) {
+        self.cssconditionrule
+            .update_rules(mediarule.rules.clone(), guard);
+        if let Some(medialist) = self.medialist.get() {
+            medialist.update_media_list(mediarule.media_queries.clone());
+        }
+        *self.mediarule.borrow_mut() = mediarule;
     }
 }
 
@@ -78,7 +93,7 @@ impl SpecificCSSRule for CSSMediaRule {
 
     fn get_css(&self) -> DOMString {
         let guard = self.cssconditionrule.shared_lock().read();
-        self.mediarule.to_css_string(&guard).into()
+        self.mediarule.borrow().to_css_string(&guard).into()
     }
 }
 

components/script/dom/cssnamespacerule.rs

@@ -2,6 +2,8 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use servo_arc::Arc;
 use style::shared_lock::ToCssWithGuard;
@@ -21,7 +23,7 @@ pub(crate) struct CSSNamespaceRule {
     cssrule: CSSRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    namespacerule: Arc<NamespaceRule>,
+    namespacerule: RefCell<Arc<NamespaceRule>>,
 }
 
 impl CSSNamespaceRule {
@@ -31,7 +33,7 @@ impl CSSNamespaceRule {
     ) -> CSSNamespaceRule {
         CSSNamespaceRule {
             cssrule: CSSRule::new_inherited(parent_stylesheet),
-            namespacerule,
+            namespacerule: RefCell::new(namespacerule),
         }
     }
 
@@ -51,12 +53,17 @@ impl CSSNamespaceRule {
             can_gc,
         )
     }
+
+    pub(crate) fn update_rule(&self, namespacerule: Arc<NamespaceRule>) {
+        *self.namespacerule.borrow_mut() = namespacerule;
+    }
 }
 
 impl CSSNamespaceRuleMethods<crate::DomTypeHolder> for CSSNamespaceRule {
     // https://drafts.csswg.org/cssom/#dom-cssnamespacerule-prefix
     fn Prefix(&self) -> DOMString {
         self.namespacerule
+            .borrow()
             .prefix
             .as_ref()
             .map(|s| s.to_string().into())
@@ -65,7 +72,7 @@ impl CSSNamespaceRuleMethods<crate::DomTypeHolder> for CSSNamespaceRule {
 
     // https://drafts.csswg.org/cssom/#dom-cssnamespacerule-namespaceuri
     fn NamespaceURI(&self) -> DOMString {
-        (**self.namespacerule.url).into()
+        (**self.namespacerule.borrow().url).into()
     }
 }
 
@@ -76,6 +83,6 @@ impl SpecificCSSRule for CSSNamespaceRule {
 
     fn get_css(&self) -> DOMString {
         let guard = self.cssrule.shared_lock().read();
-        self.namespacerule.to_css_string(&guard).into()
+        self.namespacerule.borrow().to_css_string(&guard).into()
     }
 }

components/script/dom/cssnesteddeclarations.rs

@@ -2,9 +2,11 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use servo_arc::Arc;
-use style::shared_lock::{Locked, ToCssWithGuard};
+use style::shared_lock::{Locked, SharedRwLockReadGuard, ToCssWithGuard};
 use style::stylesheets::{CssRuleType, NestedDeclarationsRule};
 
 use crate::dom::bindings::codegen::Bindings::CSSNestedDeclarationsBinding::CSSNestedDeclarationsMethods;
@@ -23,7 +25,7 @@ pub(crate) struct CSSNestedDeclarations {
     cssrule: CSSRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    nesteddeclarationsrule: Arc<Locked<NestedDeclarationsRule>>,
+    nesteddeclarationsrule: RefCell<Arc<Locked<NestedDeclarationsRule>>>,
     style_decl: MutNullableDom<CSSStyleDeclaration>,
 }
 
@@ -34,7 +36,7 @@ impl CSSNestedDeclarations {
     ) -> Self {
         Self {
             cssrule: CSSRule::new_inherited(parent_stylesheet),
-            nesteddeclarationsrule,
+            nesteddeclarationsrule: RefCell::new(nesteddeclarationsrule),
             style_decl: Default::default(),
         }
     }
@@ -55,6 +57,18 @@ impl CSSNestedDeclarations {
             can_gc,
         )
     }
+
+    pub(crate) fn update_rule(
+        &self,
+        nesteddeclarationsrule: Arc<Locked<NestedDeclarationsRule>>,
+        guard: &SharedRwLockReadGuard,
+    ) {
+        if let Some(ref style_decl) = self.style_decl.get() {
+            style_decl
+                .update_property_declaration_block(&nesteddeclarationsrule.read_with(guard).block);
+        }
+        *self.nesteddeclarationsrule.borrow_mut() = nesteddeclarationsrule;
+    }
 }
 
 impl SpecificCSSRule for CSSNestedDeclarations {
@@ -65,6 +79,7 @@ impl SpecificCSSRule for CSSNestedDeclarations {
     fn get_css(&self) -> DOMString {
         let guard = self.cssrule.shared_lock().read();
         self.nesteddeclarationsrule
+            .borrow()
             .read_with(&guard)
             .to_css_string(&guard)
             .into()
@@ -80,7 +95,13 @@ impl CSSNestedDeclarationsMethods<crate::DomTypeHolder> for CSSNestedDeclaration
                 self.global().as_window(),
                 CSSStyleOwner::CSSRule(
                     Dom::from_ref(self.upcast()),
-                    self.nesteddeclarationsrule.read_with(&guard).block.clone(),
+                    RefCell::new(
+                        self.nesteddeclarationsrule
+                            .borrow()
+                            .read_with(&guard)
+                            .block
+                            .clone(),
+                    ),
                 ),
                 None,
                 CSSModificationAccess::ReadWrite,

components/script/dom/cssrule.rs

@@ -5,7 +5,7 @@
 use std::cell::Cell;
 
 use dom_struct::dom_struct;
-use style::shared_lock::SharedRwLock;
+use style::shared_lock::{SharedRwLock, SharedRwLockReadGuard};
 use style::stylesheets::{CssRule as StyleCssRule, CssRuleType};
 
 use crate::dom::bindings::codegen::Bindings::CSSRuleBinding::CSSRuleMethods;
@@ -157,7 +157,73 @@ impl CSSRule {
     }
 
     pub(crate) fn shared_lock(&self) -> &SharedRwLock {
-        &self.parent_stylesheet.style_stylesheet().shared_lock
+        self.parent_stylesheet.shared_lock()
+    }
+
+    pub(crate) fn update_rule(&self, style_rule: &StyleCssRule, guard: &SharedRwLockReadGuard) {
+        match style_rule {
+            StyleCssRule::Import(s) => {
+                if let Some(rule) = self.downcast::<CSSImportRule>() {
+                    rule.update_rule(s.clone());
+                }
+            },
+            StyleCssRule::Style(s) => {
+                if let Some(rule) = self.downcast::<CSSStyleRule>() {
+                    rule.update_rule(s.clone(), guard);
+                }
+            },
+            StyleCssRule::FontFace(s) => {
+                if let Some(rule) = self.downcast::<CSSFontFaceRule>() {
+                    rule.update_rule(s.clone());
+                }
+            },
+            StyleCssRule::FontFeatureValues(_) => unimplemented!(),
+            StyleCssRule::CounterStyle(_) => unimplemented!(),
+            StyleCssRule::Keyframes(s) => {
+                if let Some(rule) = self.downcast::<CSSKeyframesRule>() {
+                    rule.update_rule(s.clone(), guard);
+                }
+            },
+            StyleCssRule::Media(s) => {
+                if let Some(rule) = self.downcast::<CSSMediaRule>() {
+                    rule.update_rule(s.clone(), guard);
+                }
+            },
+            StyleCssRule::Namespace(s) => {
+                if let Some(rule) = self.downcast::<CSSNamespaceRule>() {
+                    rule.update_rule(s.clone());
+                }
+            },
+            StyleCssRule::Supports(s) => {
+                if let Some(rule) = self.downcast::<CSSSupportsRule>() {
+                    rule.update_rule(s.clone(), guard);
+                }
+            },
+            StyleCssRule::Page(_) => unreachable!(),
+            StyleCssRule::Container(_) => unimplemented!(), // TODO
+            StyleCssRule::Document(_) => unimplemented!(),  // TODO
+            StyleCssRule::LayerBlock(s) => {
+                if let Some(rule) = self.downcast::<CSSLayerBlockRule>() {
+                    rule.update_rule(s.clone(), guard);
+                }
+            },
+            StyleCssRule::LayerStatement(s) => {
+                if let Some(rule) = self.downcast::<CSSLayerStatementRule>() {
+                    rule.update_rule(s.clone());
+                }
+            },
+            StyleCssRule::FontPaletteValues(_) => unimplemented!(), // TODO
+            StyleCssRule::Property(_) => unimplemented!(),          // TODO
+            StyleCssRule::Margin(_) => unimplemented!(),            // TODO
+            StyleCssRule::Scope(_) => unimplemented!(),             // TODO
+            StyleCssRule::StartingStyle(_) => unimplemented!(),     // TODO
+            StyleCssRule::PositionTry(_) => unimplemented!(),       // TODO
+            StyleCssRule::NestedDeclarations(s) => {
+                if let Some(rule) = self.downcast::<CSSNestedDeclarations>() {
+                    rule.update_rule(s.clone(), guard);
+                }
+            },
+        }
     }
 }
 

components/script/dom/cssrulelist.rs

@@ -4,9 +4,13 @@
 
 #![allow(unsafe_code)]
 
+use std::cell::RefCell;
+
 use dom_struct::dom_struct;
+use itertools::izip;
+use script_bindings::inheritance::Castable;
 use servo_arc::Arc;
-use style::shared_lock::Locked;
+use style::shared_lock::{Locked, SharedRwLockReadGuard};
 use style::stylesheets::{
     AllowImportRules, CssRuleType, CssRuleTypes, CssRules, CssRulesHelpers, KeyframesRule,
     RulesMutateError, StylesheetLoader as StyleStylesheetLoader,
@@ -44,7 +48,7 @@ pub(crate) struct CSSRuleList {
     reflector_: Reflector,
     parent_stylesheet: Dom<CSSStyleSheet>,
     #[ignore_malloc_size_of = "Arc"]
-    rules: RulesSource,
+    rules: RefCell<RulesSource>,
     dom_rules: DomRefCell<Vec<MutNullableDom<CSSRule>>>,
 }
 
@@ -78,7 +82,7 @@ impl CSSRuleList {
         CSSRuleList {
             reflector_: Reflector::new(),
             parent_stylesheet: Dom::from_ref(parent_stylesheet),
-            rules,
+            rules: RefCell::new(rules),
             dom_rules: DomRefCell::new(dom_rules),
         }
     }
@@ -107,8 +111,9 @@ impl CSSRuleList {
         parse_relative_rule_type: Option<CssRuleType>,
         can_gc: CanGc,
     ) -> Fallible<u32> {
-        let css_rules = if let RulesSource::Rules(ref rules) = self.rules {
+        self.parent_stylesheet.will_modify();
-            rules
+        let css_rules = if let RulesSource::Rules(rules) = &*self.rules.borrow() {
+            rules.clone()
         } else {
             panic!("Called insert_rule on non-CssRule-backed CSSRuleList");
         };
@@ -144,6 +149,7 @@ impl CSSRuleList {
             .map_err(Convert::convert)?;
 
         let parent_stylesheet = &*self.parent_stylesheet;
+        parent_stylesheet.will_modify();
         let dom_rule = CSSRule::new_specific(window, parent_stylesheet, new_rule, can_gc);
         self.dom_rules
             .borrow_mut()
@@ -154,10 +160,12 @@ impl CSSRuleList {
 
     /// In case of a keyframe rule, index must be valid.
     pub(crate) fn remove_rule(&self, index: u32) -> ErrorResult {
+        self.parent_stylesheet.will_modify();
+
         let index = index as usize;
         let mut guard = self.parent_stylesheet.shared_lock().write();
 
-        match self.rules {
+        match *self.rules.borrow() {
             RulesSource::Rules(ref css_rules) => {
                 css_rules
                     .write_with(&mut guard)
@@ -199,7 +207,7 @@ impl CSSRuleList {
             rule.or_init(|| {
                 let parent_stylesheet = &self.parent_stylesheet;
                 let lock = parent_stylesheet.shared_lock();
-                match self.rules {
+                match *self.rules.borrow() {
                     RulesSource::Rules(ref rules) => {
                         let rule = {
                             let guard = lock.read();
@@ -235,11 +243,48 @@ impl CSSRuleList {
     /// Should only be called for keyframes-backed rules, use insert_rule
     /// for CssRules-backed rules
     pub(crate) fn append_lazy_dom_rule(&self) {
-        if let RulesSource::Rules(..) = self.rules {
+        if let RulesSource::Rules(..) = &*self.rules.borrow() {
             panic!("Can only call append_lazy_rule with keyframes-backed CSSRules");
         }
         self.dom_rules.borrow_mut().push(MutNullableDom::new(None));
     }
+
+    pub(super) fn update_rules(&self, rules: RulesSource, guard: &SharedRwLockReadGuard) {
+        let dom_rules = self.dom_rules.borrow();
+        match rules {
+            RulesSource::Rules(ref css_rules) => {
+                if let RulesSource::Keyframes(..) = &*self.rules.borrow() {
+                    panic!("Called update_rules on non-CssRule-backed CSSRuleList with CssRules");
+                }
+
+                let css_rules_iter = css_rules.read_with(guard).0.iter();
+                for (dom_rule, css_rule) in izip!(dom_rules.iter(), css_rules_iter) {
+                    let Some(dom_rule) = dom_rule.get() else {
+                        continue;
+                    };
+                    dom_rule.update_rule(css_rule, guard);
+                }
+            },
+            RulesSource::Keyframes(ref keyframesrule) => {
+                if let RulesSource::Rules(..) = &*self.rules.borrow() {
+                    panic!("Called update_rules on CssRule-backed CSSRuleList with non-CssRules");
+                }
+
+                let keyframerules_iter = keyframesrule.read_with(guard).keyframes.iter();
+                for (dom_rule, keyframerule) in izip!(dom_rules.iter(), keyframerules_iter) {
+                    let Some(dom_rule) = dom_rule.get() else {
+                        continue;
+                    };
+                    let Some(dom_rule) = dom_rule.downcast::<CSSKeyframeRule>() else {
+                        continue;
+                    };
+                    dom_rule.update_rule(keyframerule.clone(), guard);
+                }
+            },
+        }
+
+        *self.rules.borrow_mut() = rules;
+    }
 }
 
 impl CSSRuleListMethods<crate::DomTypeHolder> for CSSRuleList {

components/script/dom/cssstyledeclaration.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 std::cell::RefCell;
 use std::cmp::Ordering;
 use std::sync::LazyLock;
 
@@ -53,7 +54,7 @@ pub(crate) enum CSSStyleOwner {
         Dom<CSSRule>,
         #[ignore_malloc_size_of = "Arc"]
         #[no_trace]
-        Arc<Locked<PropertyDeclarationBlock>>,
+        RefCell<Arc<Locked<PropertyDeclarationBlock>>>,
     ),
 }
 
@@ -119,9 +120,10 @@ impl CSSStyleOwner {
                 result
             },
             CSSStyleOwner::CSSRule(ref rule, ref pdb) => {
+                rule.parent_stylesheet().will_modify();
                 let result = {
                     let mut guard = rule.shared_lock().write();
-                    f(&mut *pdb.write_with(&mut guard), &mut changed)
+                    f(&mut *pdb.borrow().write_with(&mut guard), &mut changed)
                 };
                 if changed {
                     rule.parent_stylesheet().notify_invalidations();
@@ -152,7 +154,7 @@ impl CSSStyleOwner {
             },
             CSSStyleOwner::CSSRule(ref rule, ref pdb) => {
                 let guard = rule.shared_lock().read();
-                f(pdb.read_with(&guard))
+                f(pdb.borrow().read_with(&guard))
             },
         }
     }
@@ -265,6 +267,17 @@ impl CSSStyleDeclaration {
         )
     }
 
+    pub(crate) fn update_property_declaration_block(
+        &self,
+        pdb: &Arc<Locked<PropertyDeclarationBlock>>,
+    ) {
+        if let CSSStyleOwner::CSSRule(_, pdb_cell) = &self.owner {
+            *pdb_cell.borrow_mut() = pdb.clone();
+        } else {
+            panic!("update_rule called on CSSStyleDeclaration with a Element owner");
+        }
+    }
+
     fn get_computed_style(&self, property: PropertyId) -> DOMString {
         match self.owner {
             CSSStyleOwner::CSSRule(..) => {

components/script/dom/cssstylerule.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 std::cell::RefCell;
 use std::mem;
 
 use cssparser::{Parser as CssParser, ParserInput as CssParserInput, ToCss};
@@ -9,7 +10,7 @@ use dom_struct::dom_struct;
 use selectors::parser::{ParseRelative, SelectorList};
 use servo_arc::Arc;
 use style::selector_parser::SelectorParser;
-use style::shared_lock::{Locked, ToCssWithGuard};
+use style::shared_lock::{Locked, SharedRwLockReadGuard, ToCssWithGuard};
 use style::stylesheets::{CssRuleType, CssRules, Origin, StyleRule};
 
 use crate::dom::bindings::codegen::Bindings::CSSStyleRuleBinding::CSSStyleRuleMethods;
@@ -29,7 +30,7 @@ pub(crate) struct CSSStyleRule {
     cssgroupingrule: CSSGroupingRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    stylerule: Arc<Locked<StyleRule>>,
+    stylerule: RefCell<Arc<Locked<StyleRule>>>,
     style_decl: MutNullableDom<CSSStyleDeclaration>,
 }
 
@@ -40,7 +41,7 @@ impl CSSStyleRule {
     ) -> CSSStyleRule {
         CSSStyleRule {
             cssgroupingrule: CSSGroupingRule::new_inherited(parent_stylesheet),
-            stylerule,
+            stylerule: RefCell::new(stylerule),
             style_decl: Default::default(),
         }
     }
@@ -63,11 +64,28 @@ impl CSSStyleRule {
         let lock = self.cssgroupingrule.shared_lock();
         let mut guard = lock.write();
         self.stylerule
+            .borrow()
             .write_with(&mut guard)
             .rules
             .get_or_insert_with(|| CssRules::new(vec![], lock))
             .clone()
     }
+
+    pub(crate) fn update_rule(
+        &self,
+        stylerule: Arc<Locked<StyleRule>>,
+        guard: &SharedRwLockReadGuard,
+    ) {
+        if let Some(ref rules) = stylerule.read_with(guard).rules {
+            self.cssgroupingrule.update_rules(rules, guard);
+        }
+
+        if let Some(ref style_decl) = self.style_decl.get() {
+            style_decl.update_property_declaration_block(&stylerule.read_with(guard).block);
+        }
+
+        *self.stylerule.borrow_mut() = stylerule;
+    }
 }
 
 impl SpecificCSSRule for CSSStyleRule {
@@ -78,6 +96,7 @@ impl SpecificCSSRule for CSSStyleRule {
     fn get_css(&self) -> DOMString {
         let guard = self.cssgroupingrule.shared_lock().read();
         self.stylerule
+            .borrow()
             .read_with(&guard)
             .to_css_string(&guard)
             .into()
@@ -93,7 +112,7 @@ impl CSSStyleRuleMethods<crate::DomTypeHolder> for CSSStyleRule {
                 self.global().as_window(),
                 CSSStyleOwner::CSSRule(
                     Dom::from_ref(self.upcast()),
-                    self.stylerule.read_with(&guard).block.clone(),
+                    RefCell::new(self.stylerule.borrow().read_with(&guard).block.clone()),
                 ),
                 None,
                 CSSModificationAccess::ReadWrite,
@@ -105,8 +124,13 @@ impl CSSStyleRuleMethods<crate::DomTypeHolder> for CSSStyleRule {
     // https://drafts.csswg.org/cssom/#dom-cssstylerule-selectortext
     fn SelectorText(&self) -> DOMString {
         let guard = self.cssgroupingrule.shared_lock().read();
-        let stylerule = self.stylerule.read_with(&guard);
+        DOMString::from_string(
-        DOMString::from_string(stylerule.selectors.to_css_string())
+            self.stylerule
+                .borrow()
+                .read_with(&guard)
+                .selectors
+                .to_css_string(),
+        )
     }
 
     // https://drafts.csswg.org/cssom/#dom-cssstylerule-selectortext
@@ -115,7 +139,8 @@ impl CSSStyleRuleMethods<crate::DomTypeHolder> for CSSStyleRule {
             .cssgroupingrule
             .parent_stylesheet()
             .style_stylesheet()
-            .contents;
+            .contents
+            .clone();
         // It's not clear from the spec if we should use the stylesheet's namespaces.
         // https://github.com/w3c/csswg-drafts/issues/1511
         let namespaces = contents.namespaces.read();
@@ -131,10 +156,13 @@ impl CSSStyleRuleMethods<crate::DomTypeHolder> for CSSStyleRule {
         // TODO: Maybe allow setting relative selectors from the OM, if we're in a nested style
         // rule?
         if let Ok(mut s) = SelectorList::parse(&parser, &mut css_parser, ParseRelative::No) {
+            self.cssgroupingrule.parent_stylesheet().will_modify();
             // This mirrors what we do in CSSStyleOwner::mutate_associated_block.
             let mut guard = self.cssgroupingrule.shared_lock().write();
-            let stylerule = self.stylerule.write_with(&mut guard);
+            mem::swap(
-            mem::swap(&mut stylerule.selectors, &mut s);
+                &mut self.stylerule.borrow().write_with(&mut guard).selectors,
+                &mut s,
+            );
             self.cssgroupingrule
                 .parent_stylesheet()
                 .notify_invalidations();

components/script/dom/cssstylesheet.rs

@@ -2,16 +2,17 @@
  * 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 std::cell::Cell;
+use std::cell::{Cell, Ref};
 use std::rc::Rc;
 
 use dom_struct::dom_struct;
 use js::rust::HandleObject;
+use script_bindings::inheritance::Castable;
 use script_bindings::realms::InRealm;
 use script_bindings::root::Dom;
 use servo_arc::Arc;
 use style::media_queries::MediaList as StyleMediaList;
-use style::shared_lock::SharedRwLock;
+use style::shared_lock::{SharedRwLock, SharedRwLockReadGuard};
 use style::stylesheets::{
     AllowImportRules, CssRuleTypes, Origin, Stylesheet as StyleStyleSheet, UrlExtraData,
 };
@@ -33,6 +34,7 @@ use crate::dom::bindings::str::{DOMString, USVString};
 use crate::dom::cssrulelist::{CSSRuleList, RulesSource};
 use crate::dom::document::Document;
 use crate::dom::element::Element;
+use crate::dom::htmlstyleelement::HTMLStyleElement;
 use crate::dom::medialist::MediaList;
 use crate::dom::node::NodeTraits;
 use crate::dom::stylesheet::StyleSheet;
@@ -55,7 +57,12 @@ pub(crate) struct CSSStyleSheet {
     /// The inner Stylo's [Stylesheet].
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    style_stylesheet: Arc<StyleStyleSheet>,
+    style_stylesheet: DomRefCell<Arc<StyleStyleSheet>>,
+
+    /// The inner Stylo's [SharedRwLock], stored at here to avoid referencing
+    /// temporary variables.
+    #[no_trace]
+    style_shared_lock: SharedRwLock,
 
     /// <https://drafts.csswg.org/cssom/#concept-css-style-sheet-origin-clean-flag>
     origin_clean: Cell<bool>,
@@ -86,7 +93,8 @@ impl CSSStyleSheet {
             stylesheet: StyleSheet::new_inherited(type_, href, title),
             owner_node: MutNullableDom::new(owner),
             rulelist: MutNullableDom::new(None),
-            style_stylesheet: stylesheet,
+            style_shared_lock: stylesheet.shared_lock.clone(),
+            style_stylesheet: DomRefCell::new(stylesheet),
             origin_clean: Cell::new(true),
             constructor_document: constructor_document.map(Dom::from_ref),
             adopters: Default::default(),
@@ -150,7 +158,7 @@ impl CSSStyleSheet {
 
     fn rulelist(&self, can_gc: CanGc) -> DomRoot<CSSRuleList> {
         self.rulelist.or_init(|| {
-            let rules = self.style_stylesheet.contents.rules.clone();
+            let rules = self.style_stylesheet.borrow().contents.rules.clone();
             CSSRuleList::new(
                 self.global().as_window(),
                 self,
@@ -161,7 +169,7 @@ impl CSSStyleSheet {
     }
 
     pub(crate) fn disabled(&self) -> bool {
-        self.style_stylesheet.disabled()
+        self.style_stylesheet.borrow().disabled()
     }
 
     pub(crate) fn owner_node(&self) -> Option<DomRoot<Element>> {
@@ -169,7 +177,7 @@ impl CSSStyleSheet {
     }
 
     pub(crate) fn set_disabled(&self, disabled: bool) {
-        if self.style_stylesheet.set_disabled(disabled) {
+        if self.style_stylesheet.borrow().set_disabled(disabled) {
             self.notify_invalidations();
         }
     }
@@ -179,15 +187,11 @@ impl CSSStyleSheet {
     }
 
     pub(crate) fn shared_lock(&self) -> &SharedRwLock {
-        &self.style_stylesheet.shared_lock
+        &self.style_shared_lock
     }
 
-    pub(crate) fn style_stylesheet(&self) -> &StyleStyleSheet {
+    pub(crate) fn style_stylesheet(&self) -> Ref<'_, Arc<StyleStyleSheet>> {
-        &self.style_stylesheet
+        self.style_stylesheet.borrow()
-    }
-
-    pub(crate) fn style_stylesheet_arc(&self) -> &Arc<StyleStyleSheet> {
-        &self.style_stylesheet
     }
 
     pub(crate) fn set_origin_clean(&self, origin_clean: bool) {
@@ -231,6 +235,36 @@ impl CSSStyleSheet {
         }
     }
 
+    pub(crate) fn will_modify(&self) {
+        let Some(node) = self.owner_node.get() else {
+            return;
+        };
+
+        let Some(node) = node.downcast::<HTMLStyleElement>() else {
+            return;
+        };
+
+        node.will_modify_stylesheet();
+    }
+
+    pub(crate) fn update_style_stylesheet(
+        &self,
+        style_stylesheet: &Arc<StyleStyleSheet>,
+        guard: &SharedRwLockReadGuard,
+    ) {
+        // When the shared `StylesheetContents` is about to be modified,
+        // `CSSStyleSheet::owner_node` performs a copy-on-write to avoid
+        // affecting other sharers, see `CSSStyleSheet::will_modify`. And
+        // then updates the references to `CssRule` or `PropertyDeclarationBlock`
+        // stored in the CSSOMs to ensure that modifications are made only
+        // on the new copy.
+        *self.style_stylesheet.borrow_mut() = style_stylesheet.clone();
+        if let Some(rulelist) = self.rulelist.get() {
+            let rules = style_stylesheet.contents.rules.clone();
+            rulelist.update_rules(RulesSource::Rules(rules), guard);
+        }
+    }
+
     /// Invalidate all stylesheet set this stylesheet is a part on.
     pub(crate) fn notify_invalidations(&self) {
         if let Some(owner) = self.owner_node() {
@@ -402,8 +436,12 @@ impl CSSStyleSheetMethods<crate::DomTypeHolder> for CSSStyleSheet {
                 let global = sheet.global();
                 let window = global.as_window();
 
+                sheet.will_modify();
+
+                #[cfg(feature = "tracing")]
+                let _span = tracing::trace_span!("ParseStylesheet", servo_profiling = true).entered();
                 StyleStyleSheet::update_from_str(
-                    &sheet.style_stylesheet,
+                    &sheet.style_stylesheet(),
                     &text,
                     UrlExtraData(window.get_url().get_arc()),
                     None,
@@ -441,8 +479,12 @@ impl CSSStyleSheetMethods<crate::DomTypeHolder> for CSSStyleSheet {
         let global = self.global();
         let window = global.as_window();
 
+        self.will_modify();
+
+        #[cfg(feature = "tracing")]
+        let _span = tracing::trace_span!("ParseStylesheet", servo_profiling = true).entered();
         StyleStyleSheet::update_from_str(
-            &self.style_stylesheet,
+            &self.style_stylesheet(),
             &text,
             UrlExtraData(window.get_url().get_arc()),
             None,

components/script/dom/csssupportsrule.rs

@@ -2,9 +2,11 @@
  * 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 std::cell::RefCell;
+
 use dom_struct::dom_struct;
 use servo_arc::Arc;
-use style::shared_lock::ToCssWithGuard;
+use style::shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
 use style::stylesheets::{CssRuleType, SupportsRule};
 use style_traits::ToCss;
 
@@ -22,7 +24,7 @@ pub(crate) struct CSSSupportsRule {
     cssconditionrule: CSSConditionRule,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    supportsrule: Arc<SupportsRule>,
+    supportsrule: RefCell<Arc<SupportsRule>>,
 }
 
 impl CSSSupportsRule {
@@ -33,7 +35,7 @@ impl CSSSupportsRule {
         let list = supportsrule.rules.clone();
         CSSSupportsRule {
             cssconditionrule: CSSConditionRule::new_inherited(parent_stylesheet, list),
-            supportsrule,
+            supportsrule: RefCell::new(supportsrule),
         }
     }
 
@@ -56,7 +58,17 @@ impl CSSSupportsRule {
 
     /// <https://drafts.csswg.org/css-conditional-3/#the-csssupportsrule-interface>
     pub(crate) fn get_condition_text(&self) -> DOMString {
-        self.supportsrule.condition.to_css_string().into()
+        self.supportsrule.borrow().condition.to_css_string().into()
+    }
+
+    pub(crate) fn update_rule(
+        &self,
+        supportsrule: Arc<SupportsRule>,
+        guard: &SharedRwLockReadGuard,
+    ) {
+        self.cssconditionrule
+            .update_rules(supportsrule.rules.clone(), guard);
+        *self.supportsrule.borrow_mut() = supportsrule;
     }
 }
 
@@ -67,6 +79,6 @@ impl SpecificCSSRule for CSSSupportsRule {
 
     fn get_css(&self) -> DOMString {
         let guard = self.cssconditionrule.shared_lock().read();
-        self.supportsrule.to_css_string(&guard).into()
+        self.supportsrule.borrow().to_css_string(&guard).into()
     }
 }

components/script/dom/document.rs

@@ -4021,7 +4021,7 @@ impl Document {
         debug_assert!(cssom_stylesheet.is_constructed());
 
         let stylesheets = &mut *self.stylesheets.borrow_mut();
-        let sheet = cssom_stylesheet.style_stylesheet_arc().clone();
+        let sheet = cssom_stylesheet.style_stylesheet().clone();
 
         let insertion_point = stylesheets
             .iter()

components/script/dom/documentorshadowroot.rs

@@ -397,7 +397,7 @@ impl DocumentOrShadowRoot {
             if stylesheet_remove_set.insert(sheet_to_remove) {
                 owner.remove_stylesheet(
                     StylesheetSource::Constructed(sheet_to_remove.clone()),
-                    sheet_to_remove.style_stylesheet_arc(),
+                    &sheet_to_remove.style_stylesheet(),
                 );
                 sheet_to_remove.remove_adopter(owner);
             }
@@ -416,7 +416,7 @@ impl DocumentOrShadowRoot {
                 // around.
                 owner.remove_stylesheet(
                     StylesheetSource::Constructed(sheet.clone()),
-                    sheet.style_stylesheet_arc(),
+                    &sheet.style_stylesheet(),
                 );
             } else {
                 sheet.add_adopter(owner.clone());

components/script/dom/htmlstyleelement.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use std::cell::Cell;
+use std::sync::atomic::{AtomicBool, Ordering};
 
 use dom_struct::dom_struct;
 use html5ever::{LocalName, Prefix};
@@ -11,7 +12,8 @@ use net_traits::ReferrerPolicy;
 use script_bindings::root::Dom;
 use servo_arc::Arc;
 use style::media_queries::MediaList as StyleMediaList;
-use style::stylesheets::{AllowImportRules, Origin, Stylesheet, UrlExtraData};
+use style::shared_lock::DeepCloneWithLock;
+use style::stylesheets::{AllowImportRules, Origin, Stylesheet, StylesheetContents, UrlExtraData};
 
 use crate::dom::attr::Attr;
 use crate::dom::bindings::cell::DomRefCell;
@@ -29,6 +31,7 @@ use crate::dom::htmlelement::HTMLElement;
 use crate::dom::medialist::MediaList;
 use crate::dom::node::{BindContext, ChildrenMutation, Node, NodeTraits, UnbindContext};
 use crate::dom::stylesheet::StyleSheet as DOMStyleSheet;
+use crate::dom::stylesheetcontentscache::{StylesheetContentsCache, StylesheetContentsCacheKey};
 use crate::dom::virtualmethods::VirtualMethods;
 use crate::script_runtime::CanGc;
 use crate::stylesheet_loader::{StylesheetLoader, StylesheetOwner};
@@ -39,6 +42,8 @@ pub(crate) struct HTMLStyleElement {
     #[conditional_malloc_size_of]
     #[no_trace]
     stylesheet: DomRefCell<Option<Arc<Stylesheet>>>,
+    #[no_trace]
+    stylesheetcontents_cache_key: DomRefCell<Option<StylesheetContentsCacheKey>>,
     cssom_stylesheet: MutNullableDom<CSSStyleSheet>,
     /// <https://html.spec.whatwg.org/multipage/#a-style-sheet-that-is-blocking-scripts>
     parser_inserted: Cell<bool>,
@@ -57,6 +62,7 @@ impl HTMLStyleElement {
         HTMLStyleElement {
             htmlelement: HTMLElement::new_inherited(local_name, prefix, document),
             stylesheet: DomRefCell::new(None),
+            stylesheetcontents_cache_key: DomRefCell::new(None),
             cssom_stylesheet: MutNullableDom::new(None),
             parser_inserted: Cell::new(creator.is_parser_created()),
             in_stack_of_open_elements: Cell::new(creator.is_parser_created()),
@@ -125,19 +131,41 @@ impl HTMLStyleElement {
         let shared_lock = node.owner_doc().style_shared_lock().clone();
         let mq = Arc::new(shared_lock.wrap(self.create_media_list(&self.Media())));
         let loader = StylesheetLoader::for_element(self.upcast());
-        let sheet = Stylesheet::from_str(
+
+        let stylesheetcontents_create_callback = || {
+            #[cfg(feature = "tracing")]
+            let _span = tracing::trace_span!("ParseStylesheet", servo_profiling = true).entered();
+            StylesheetContents::from_str(
                 &data,
                 UrlExtraData(window.get_url().get_arc()),
                 Origin::Author,
-            mq,
+                &shared_lock,
-            shared_lock,
                 Some(&loader),
                 window.css_error_reporter(),
                 doc.quirks_mode(),
                 AllowImportRules::Yes,
+                /* sanitized_output = */ None,
+            )
+        };
+
+        // For duplicate style sheets with identical content, `StylesheetContents` can be reused
+        // to avoid reedundant parsing of the style sheets. Additionally, the cache hit rate of
+        // stylo's `CascadeDataCache` can now be significantly improved. When shared `StylesheetContents`
+        // is modified, copy-on-write will occur, see `CSSStyleSheet::will_modify`.
+        let (cache_key, contents) = StylesheetContentsCache::get_or_insert_with(
+            &data,
+            &shared_lock,
+            UrlExtraData(window.get_url().get_arc()),
+            doc.quirks_mode(),
+            stylesheetcontents_create_callback,
         );
 
-        let sheet = Arc::new(sheet);
+        let sheet = Arc::new(Stylesheet {
+            contents,
+            shared_lock,
+            media: mq,
+            disabled: AtomicBool::new(false),
+        });
 
         // No subresource loads were triggered, queue load event
         if self.pending_loads.get() == 0 {
@@ -147,22 +175,44 @@ impl HTMLStyleElement {
                 .queue_simple_event(self.upcast(), atom!("load"));
         }
 
-        self.set_stylesheet(sheet);
+        self.set_stylesheet(sheet, cache_key, true);
     }
 
     // FIXME(emilio): This is duplicated with HTMLLinkElement::set_stylesheet.
+    //
+    // With the reuse of `StylesheetContent` for same stylesheet string content,
+    // this function has a bit difference with `HTMLLinkElement::set_stylesheet` now.
     #[cfg_attr(crown, allow(crown::unrooted_must_root))]
-    pub(crate) fn set_stylesheet(&self, s: Arc<Stylesheet>) {
+    pub(crate) fn set_stylesheet(
+        &self,
+        s: Arc<Stylesheet>,
+        cache_key: Option<StylesheetContentsCacheKey>,
+        need_clean_cssom: bool,
+    ) {
         let stylesheets_owner = self.stylesheet_list_owner();
         if let Some(ref s) = *self.stylesheet.borrow() {
             stylesheets_owner
-                .remove_stylesheet(StylesheetSource::Element(Dom::from_ref(self.upcast())), s)
+                .remove_stylesheet(StylesheetSource::Element(Dom::from_ref(self.upcast())), s);
         }
-        *self.stylesheet.borrow_mut() = Some(s.clone());
+
+        if need_clean_cssom {
             self.clean_stylesheet_ownership();
+        } else if let Some(cssom_stylesheet) = self.cssom_stylesheet.get() {
+            let guard = s.shared_lock.read();
+            cssom_stylesheet.update_style_stylesheet(&s, &guard);
+        }
+
+        *self.stylesheet.borrow_mut() = Some(s.clone());
+        *self.stylesheetcontents_cache_key.borrow_mut() = cache_key;
         stylesheets_owner.add_owned_stylesheet(self.upcast(), s);
     }
 
+    pub(crate) fn will_modify_stylesheet(&self) {
+        if let Some(stylesheet_with_owned_contents) = self.create_owned_contents_stylesheet() {
+            self.set_stylesheet(stylesheet_with_owned_contents, None, false);
+        }
+    }
+
     pub(crate) fn get_stylesheet(&self) -> Option<Arc<Stylesheet>> {
         self.stylesheet.borrow().clone()
     }
@@ -184,18 +234,55 @@ impl HTMLStyleElement {
         })
     }
 
+    fn create_owned_contents_stylesheet(&self) -> Option<Arc<Stylesheet>> {
+        let cache_key = self.stylesheetcontents_cache_key.borrow_mut().take()?;
+        if cache_key.is_uniquely_owned() {
+            StylesheetContentsCache::remove(cache_key);
+            return None;
+        }
+
+        let stylesheet_with_shared_contents = self.stylesheet.borrow().clone()?;
+        let lock = stylesheet_with_shared_contents.shared_lock.clone();
+        let guard = stylesheet_with_shared_contents.shared_lock.read();
+        let stylesheet_with_owned_contents = Arc::new(Stylesheet {
+            contents: Arc::new(
+                stylesheet_with_shared_contents
+                    .contents
+                    .deep_clone_with_lock(&lock, &guard),
+            ),
+            shared_lock: lock,
+            media: stylesheet_with_shared_contents.media.clone(),
+            disabled: AtomicBool::new(
+                stylesheet_with_shared_contents
+                    .disabled
+                    .load(Ordering::SeqCst),
+            ),
+        });
+
+        Some(stylesheet_with_owned_contents)
+    }
+
     fn clean_stylesheet_ownership(&self) {
         if let Some(cssom_stylesheet) = self.cssom_stylesheet.get() {
+            // If the CSSOMs change from having an owner node to being ownerless, they may still
+            // potentially modify shared stylesheets. Thus, create an new `Stylesheet` with owned
+            // `StylesheetContents` to ensure that the potentially modifications are only made on
+            // the owned `StylesheetContents`.
+            if let Some(stylesheet) = self.create_owned_contents_stylesheet() {
+                let guard = stylesheet.shared_lock.read();
+                cssom_stylesheet.update_style_stylesheet(&stylesheet, &guard);
+            }
             cssom_stylesheet.set_owner_node(None);
         }
         self.cssom_stylesheet.set(None);
     }
 
     fn remove_stylesheet(&self) {
-        if let Some(s) = self.stylesheet.borrow_mut().take() {
         self.clean_stylesheet_ownership();
+        if let Some(s) = self.stylesheet.borrow_mut().take() {
             self.stylesheet_list_owner()
-                .remove_stylesheet(StylesheetSource::Element(Dom::from_ref(self.upcast())), &s)
+                .remove_stylesheet(StylesheetSource::Element(Dom::from_ref(self.upcast())), &s);
+            let _ = self.stylesheetcontents_cache_key.borrow_mut().take();
         }
     }
 }

components/script/dom/medialist.rs

@@ -2,6 +2,8 @@
  * 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 std::cell::RefCell;
+
 use cssparser::{Parser, ParserInput};
 use dom_struct::dom_struct;
 use servo_arc::Arc;
@@ -26,7 +28,7 @@ pub(crate) struct MediaList {
     parent_stylesheet: Dom<CSSStyleSheet>,
     #[ignore_malloc_size_of = "Arc"]
     #[no_trace]
-    media_queries: Arc<Locked<StyleMediaList>>,
+    media_queries: RefCell<Arc<Locked<StyleMediaList>>>,
 }
 
 impl MediaList {
@@ -38,7 +40,7 @@ impl MediaList {
         MediaList {
             parent_stylesheet: Dom::from_ref(parent_stylesheet),
             reflector_: Reflector::new(),
-            media_queries,
+            media_queries: RefCell::new(media_queries),
         }
     }
 
@@ -57,7 +59,7 @@ impl MediaList {
     }
 
     fn shared_lock(&self) -> &SharedRwLock {
-        &self.parent_stylesheet.style_stylesheet().shared_lock
+        self.parent_stylesheet.shared_lock()
     }
 
     /// <https://drafts.csswg.org/cssom/#parse-a-media-query-list>
@@ -109,7 +111,11 @@ impl MediaList {
 
     pub(crate) fn clone_media_list(&self) -> StyleMediaList {
         let guard = self.shared_lock().read();
-        self.media_queries.read_with(&guard).clone()
+        self.media_queries.borrow().read_with(&guard).clone()
+    }
+
+    pub(crate) fn update_media_list(&self, media_queries: Arc<Locked<StyleMediaList>>) {
+        *self.media_queries.borrow_mut() = media_queries;
     }
 }
 
@@ -117,14 +123,21 @@ impl MediaListMethods<crate::DomTypeHolder> for MediaList {
     /// <https://drafts.csswg.org/cssom/#dom-medialist-mediatext>
     fn MediaText(&self) -> DOMString {
         let guard = self.shared_lock().read();
-        DOMString::from(self.media_queries.read_with(&guard).to_css_string())
+        DOMString::from(
+            self.media_queries
+                .borrow()
+                .read_with(&guard)
+                .to_css_string(),
+        )
     }
 
     /// <https://drafts.csswg.org/cssom/#dom-medialist-mediatext>
     fn SetMediaText(&self, value: DOMString) {
+        self.parent_stylesheet.will_modify();
         let global = self.global();
         let mut guard = self.shared_lock().write();
-        let media_queries = self.media_queries.write_with(&mut guard);
+        let media_queries_borrowed = self.media_queries.borrow();
+        let media_queries = media_queries_borrowed.write_with(&mut guard);
         *media_queries = Self::parse_media_list(&value, global.as_window());
         self.parent_stylesheet.notify_invalidations();
     }
@@ -132,13 +145,18 @@ impl MediaListMethods<crate::DomTypeHolder> for MediaList {
     // https://drafts.csswg.org/cssom/#dom-medialist-length
     fn Length(&self) -> u32 {
         let guard = self.shared_lock().read();
-        self.media_queries.read_with(&guard).media_queries.len() as u32
+        self.media_queries
+            .borrow()
+            .read_with(&guard)
+            .media_queries
+            .len() as u32
     }
 
     /// <https://drafts.csswg.org/cssom/#dom-medialist-item>
     fn Item(&self, index: u32) -> Option<DOMString> {
         let guard = self.shared_lock().read();
         self.media_queries
+            .borrow()
             .read_with(&guard)
             .media_queries
             .get(index as usize)
@@ -160,18 +178,28 @@ impl MediaListMethods<crate::DomTypeHolder> for MediaList {
             return;
         }
         // Step 3
+        {
             let m_serialized = m.clone().unwrap().to_css_string();
-        let mut guard = self.shared_lock().write();
+            let guard = self.shared_lock().read();
-        let mq = self.media_queries.write_with(&mut guard);
+            let any = self
-        let any = mq
+                .media_queries
+                .borrow()
+                .read_with(&guard)
                 .media_queries
                 .iter()
                 .any(|q| m_serialized == q.to_css_string());
             if any {
                 return;
             }
+        }
         // Step 4
-        mq.media_queries.push(m.unwrap());
+        self.parent_stylesheet.will_modify();
+        let mut guard = self.shared_lock().write();
+        self.media_queries
+            .borrow()
+            .write_with(&mut guard)
+            .media_queries
+            .push(m.unwrap());
         self.parent_stylesheet.notify_invalidations();
     }
 
@@ -185,9 +213,11 @@ impl MediaListMethods<crate::DomTypeHolder> for MediaList {
             return;
         }
         // Step 3
+        self.parent_stylesheet.will_modify();
         let m_serialized = m.unwrap().to_css_string();
         let mut guard = self.shared_lock().write();
-        let media_list = self.media_queries.write_with(&mut guard);
+        let media_queries_borrowed = self.media_queries.borrow();
+        let media_list = media_queries_borrowed.write_with(&mut guard);
         let new_vec = media_list
             .media_queries
             .drain(..)

components/script/dom/mod.rs

@@ -562,6 +562,7 @@ pub(crate) mod storage;
 pub(crate) mod storageevent;
 pub(crate) mod stylepropertymapreadonly;
 pub(crate) mod stylesheet;
+pub(crate) mod stylesheetcontentscache;
 pub(crate) mod stylesheetlist;
 pub(crate) mod submitevent;
 pub(crate) mod subtlecrypto;

components/script/dom/shadowroot.rs

@@ -242,7 +242,7 @@ impl ShadowRoot {
         debug_assert!(cssom_stylesheet.is_constructed());
 
         let stylesheets = &mut self.author_styles.borrow_mut().stylesheets;
-        let sheet = cssom_stylesheet.style_stylesheet_arc().clone();
+        let sheet = cssom_stylesheet.style_stylesheet().clone();
 
         let insertion_point = stylesheets.iter().last().cloned();
 

components/script/dom/stylesheetcontentscache.rs

@@ -0,0 +1,122 @@
+/* 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/. */
+
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::collections::hash_map::Entry;
+use std::hash::{DefaultHasher, Hash, Hasher};
+use std::rc::Rc;
+
+use servo_arc::Arc as ServoArc;
+use style::context::QuirksMode;
+use style::shared_lock::SharedRwLock;
+use style::stylesheets::{CssRule, StylesheetContents, UrlExtraData};
+use stylo_atoms::Atom;
+
+const MAX_LENGTH_OF_TEXT_INSERTED_INTO_TABLE: usize = 1024;
+const UNIQUE_OWNED: usize = 2;
+
+/// Using [`Atom`] as a cache key to avoid inefficient string content comparison. Although
+/// the [`Atom`] is already based on reference counting, an extra [`Rc`] is introduced
+/// to trace how many [`style::stylesheets::Stylesheet`]s of style elements are sharing
+/// same [`StylesheetContents`], based on the following considerations:
+/// * The reference count within [`Atom`] is dedicated to lifecycle management and is not
+///   suitable for tracking the number of [`StylesheetContents`]s owners.
+/// * The reference count within [`Atom`] is not publicly acessible.
+#[derive(Clone, Eq, Hash, MallocSizeOf, PartialEq)]
+pub(crate) struct StylesheetContentsCacheKey {
+    #[conditional_malloc_size_of]
+    stylesheet_text: Rc<Atom>,
+    base_url: Atom,
+    #[ignore_malloc_size_of = "defined in style crate"]
+    quirks_mode: QuirksMode,
+}
+
+impl StylesheetContentsCacheKey {
+    fn new(stylesheet_text: &str, base_url: &str, quirks_mode: QuirksMode) -> Self {
+        // The stylesheet text may be quite lengthy, exceeding hundreds of kilobytes.
+        // Instead of directly inserting such a huge string into AtomicString table,
+        // take its hash value and use that. (This is not a cryptographic hash, so a
+        // page could cause collisions if it wanted to.)
+        let contents_atom = if stylesheet_text.len() > MAX_LENGTH_OF_TEXT_INSERTED_INTO_TABLE {
+            let mut hasher = DefaultHasher::new();
+            stylesheet_text.hash(&mut hasher);
+            Atom::from(hasher.finish().to_string().as_str())
+        } else {
+            Atom::from(stylesheet_text)
+        };
+
+        Self {
+            stylesheet_text: Rc::new(contents_atom),
+            base_url: Atom::from(base_url),
+            quirks_mode,
+        }
+    }
+
+    pub(crate) fn is_uniquely_owned(&self) -> bool {
+        // The cache itself already holds one reference.
+        Rc::strong_count(&self.stylesheet_text) <= UNIQUE_OWNED
+    }
+}
+
+thread_local! {
+    static STYLESHEETCONTENTS_CACHE: RefCell<HashMap<StylesheetContentsCacheKey, ServoArc<StylesheetContents>>> =
+       RefCell::default();
+}
+
+pub(crate) struct StylesheetContentsCache;
+
+impl StylesheetContentsCache {
+    fn contents_can_be_cached(contents: &StylesheetContents, shared_lock: &SharedRwLock) -> bool {
+        let guard = shared_lock.read();
+        let rules = contents.rules(&guard);
+        // The copy-on-write can not be performed when the modification happens on the
+        // imported stylesheet, because it containing cssom has no owner dom node.
+        !(rules.is_empty() || rules.iter().any(|rule| matches!(rule, CssRule::Import(_))))
+    }
+
+    pub(crate) fn get_or_insert_with(
+        stylesheet_text: &str,
+        shared_lock: &SharedRwLock,
+        url_data: UrlExtraData,
+        quirks_mode: QuirksMode,
+        stylesheetcontents_create_callback: impl FnOnce() -> ServoArc<StylesheetContents>,
+    ) -> (
+        Option<StylesheetContentsCacheKey>,
+        ServoArc<StylesheetContents>,
+    ) {
+        let cache_key =
+            StylesheetContentsCacheKey::new(stylesheet_text, url_data.as_str(), quirks_mode);
+        STYLESHEETCONTENTS_CACHE.with_borrow_mut(|stylesheetcontents_cache| {
+            let entry = stylesheetcontents_cache.entry(cache_key);
+            match entry {
+                Entry::Occupied(occupied_entry) => {
+                    // Use a copy of the cache key from `Entry` instead of the newly created one above
+                    // to correctly update and track to owner count of `StylesheetContents`.
+                    (
+                        Some(occupied_entry.key().clone()),
+                        occupied_entry.get().clone(),
+                    )
+                },
+                Entry::Vacant(vacant_entry) => {
+                    let contents = stylesheetcontents_create_callback();
+                    if Self::contents_can_be_cached(&contents, shared_lock) {
+                        let occupied_entry = vacant_entry.insert_entry(contents.clone());
+                        // Use a copy of the cache key from `Entry` instead of the newly created one above
+                        // to correctly update and track to owner count of `StylesheetContents`.
+                        (Some(occupied_entry.key().clone()), contents)
+                    } else {
+                        (None, contents)
+                    }
+                },
+            }
+        })
+    }
+
+    pub(crate) fn remove(cache_key: StylesheetContentsCacheKey) {
+        STYLESHEETCONTENTS_CACHE.with_borrow_mut(|stylesheetcontents_cache| {
+            stylesheetcontents_cache.remove(&cache_key)
+        });
+    }
+}

components/script/stylesheet_loader.rs

@@ -214,6 +214,9 @@ impl FetchResponseListener for StylesheetContext {
                         .is_none_or(|generation| generation == link.get_request_generation_id());
                     if is_stylesheet_load_applicable {
                         let shared_lock = document.style_shared_lock().clone();
+                        #[cfg(feature = "tracing")]
+                        let _span = tracing::trace_span!("ParseStylesheet", servo_profiling = true)
+                            .entered();
                         let sheet = Arc::new(Stylesheet::from_bytes(
                             &data,
                             UrlExtraData(final_url.get_arc()),
@@ -235,6 +238,9 @@ impl FetchResponseListener for StylesheetContext {
                     }
                 },
                 StylesheetContextSource::Import(ref stylesheet) => {
+                    #[cfg(feature = "tracing")]
+                    let _span =
+                        tracing::trace_span!("ParseStylesheet", servo_profiling = true).entered();
                     Stylesheet::update_from_bytes(
                         stylesheet,
                         &data,