CSS animations should participate in the cascade

https://bugs.webkit.org/show_bug.cgi?id=210963
rdar://62301534

Reviewed by Antoine Quint.

Property animation can affect other properties, for example via em units when animating font-size or var() when animating custom properties.
Animated values can also be overridden by !important properties.

With this patch we start applying the cascade after computing animated style. Only the properties that may be affected are recomputed.

* LayoutTests/TestExpectations:
* LayoutTests/imported/w3c/web-platform-tests/css/css-animations/animation-base-response-001-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-animations/animation-base-response-003-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-animations/animation-base-response-004-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-animations/animation-important-001-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-animations/keyframes-unrelated-custom-property-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-properties-values-api/animation/custom-property-animation-non-inherited-used-by-standard-property-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-properties-values-api/animation/custom-property-transition-non-inherited-used-by-standard-property-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-transitions/transition-base-response-001-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-transitions/transition-base-response-003-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-variables/variable-animation-from-to-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-variables/variable-animation-over-transition-expected.txt:

A pile of progressions.

* Source/WebCore/animation/KeyframeEffectStack.cpp:
(WebCore::KeyframeEffectStack::applyKeyframeEffects):

Collected animated properties.

* Source/WebCore/animation/KeyframeEffectStack.h:
* Source/WebCore/animation/WebAnimationTypes.h:
* Source/WebCore/dom/Element.h:
* Source/WebCore/html/shadow/TextControlInnerElements.cpp:
(WebCore::TextControlInnerElement::resolveCustomStyle):
(WebCore::TextControlInnerTextElement::resolveCustomStyle):
* Source/WebCore/rendering/style/RenderStyle.cpp:
(WebCore::RenderStyle::customPropertiesEqual const):
* Source/WebCore/rendering/style/RenderStyle.h:
* Source/WebCore/style/ElementRuleCollector.cpp:
(WebCore::Style::ElementRuleCollector::ElementRuleCollector):

Switch to std::unique_ptr<MatchResult> for easier shuffling around.

(WebCore::Style::ElementRuleCollector::matchResult const):
(WebCore::Style::ElementRuleCollector::releaseMatchResult):
(WebCore::Style::ElementRuleCollector::addElementStyleProperties):
(WebCore::Style::ElementRuleCollector::declarationsForOrigin):
(WebCore::Style::ElementRuleCollector::transferMatchedRules):
(WebCore::Style::ElementRuleCollector::addMatchedProperties):
(WebCore::Style::ElementRuleCollector::addAuthorKeyframeRules):
* Source/WebCore/style/ElementRuleCollector.h:
* Source/WebCore/style/MatchResult.h:
* Source/WebCore/style/PropertyCascade.cpp:
(WebCore::Style::PropertyCascade::PropertyCascade):
(WebCore::Style::PropertyCascade::AnimationLayer::AnimationLayer):

Animations behave like a cascade layer, that's where the name of this helper struct comes from.

(WebCore::Style::PropertyCascade::set):
(WebCore::Style::PropertyCascade::setDeferred):
(WebCore::Style::PropertyCascade::addMatch):
(WebCore::Style::PropertyCascade::shouldApplyAfterAnimation):

Build a cascade that only contains properties that require re-resolving after animation.

(WebCore::Style::initializeCSSValue): Deleted.
* Source/WebCore/style/PropertyCascade.h:
(WebCore::Style::PropertyCascade::isEmpty const):

In normal cases there is nothing to apply after animations so we can bail out.

* Source/WebCore/style/StyleAdjuster.h:
* Source/WebCore/style/StyleBuilder.cpp:
(WebCore::Style::Builder::Builder):
(WebCore::Style::Builder::applyAllProperties):
* Source/WebCore/style/StyleBuilder.h:
* Source/WebCore/style/StyleResolver.cpp:
(WebCore::Style::Resolver::styleForElement):

Return the MatchResult along with the style. This allows us to reapply the cascade without matching the selectors again.

* Source/WebCore/style/StyleResolver.h:
(WebCore::Style::ElementStyle::ElementStyle): Deleted.
* Source/WebCore/style/StyleTreeResolver.cpp:
(WebCore::Style::TreeResolver::styleForStyleable):
(WebCore::Style::TreeResolver::resolveElement):
(WebCore::Style::TreeResolver::resolvePseudoElement):
(WebCore::Style::TreeResolver::resolveAncestorPseudoElement):
(WebCore::Style::TreeResolver::createAnimatedElementUpdate):
(WebCore::Style::TreeResolver::applyCascadeAfterAnimation):

Build and apply the cascade.

* Source/WebCore/style/StyleTreeResolver.h:
* Source/WebCore/style/Styleable.h:
(WebCore::Styleable::applyKeyframeEffects const):

Canonical link: https://commits.webkit.org/258514@main

LayoutTests/TestExpectations

@@ -2309,9 +2309,6 @@ webkit.org/b/148026 [ Debug ] animations/restart-after-scroll.html [ Skip ]
 
 webkit.org/b/148650 fast/repaint/add-table-overpaint.html [ Pass Failure ]
 
-imported/w3c/web-platform-tests/css/css-cascade/important-prop.html [ ImageOnlyFailure ]
-webkit.org/b/192334 imported/w3c/web-platform-tests/css/css-cascade/revert-layer-011.html [ ImageOnlyFailure ]
-
 # Initial failures on the import of css-content
 
 imported/w3c/web-platform-tests/css/css-content/element-replacement-on-replaced-element.tentative.html [ ImageOnlyFailure ]
@@ -4134,7 +4131,6 @@ webkit.org/b/206579 [ Debug ] imported/w3c/web-platform-tests/css/css-background
 
 webkit.org/b/207262 imported/w3c/web-platform-tests/web-animations/timing-model/animations/sync-start-times.html [ Pass ImageOnlyFailure ]
 
-webkit.org/b/210963 imported/w3c/web-platform-tests/css/css-animations/animation-important-002.html [ ImageOnlyFailure ]
 webkit.org/b/235110 imported/w3c/web-platform-tests/css/css-animations/flip-running-animation-via-variable.html [ ImageOnlyFailure ]
 
 webkit.org/b/230080 imported/w3c/web-platform-tests/css/css-transforms/transform-transformed-tr-percent-height-child.html [ ImageOnlyFailure ]

LayoutTests/imported/w3c/web-platform-tests/css/css-animations/animation-base-response-001-expected.txt

@@ -1,5 +1,5 @@
 
-FAIL em units respond to font-size animation assert_equals: expected "15px" but got "1px"
-FAIL ex units respond to font-size animation assert_equals: expected "6.71875px" but got "0.4375px"
-FAIL var() references respond to custom property animation assert_equals: expected "20px" but got "0px"
+PASS em units respond to font-size animation
+PASS ex units respond to font-size animation
+PASS var() references respond to custom property animation
 

LayoutTests/imported/w3c/web-platform-tests/css/css-animations/animation-base-response-003-expected.txt

@@ -1,3 +1,3 @@
 
-FAIL Identical elements are all responsive to font-size animation assert_equals: expected "15px" but got "1px"
+PASS Identical elements are all responsive to font-size animation
 

LayoutTests/imported/w3c/web-platform-tests/css/css-animations/animation-base-response-004-expected.txt

@@ -1,3 +1,3 @@
 
-FAIL Base is responsive to font-affecting appearing via setKeyframes assert_equals: expected "15px" but got "10px"
+PASS Base is responsive to font-affecting appearing via setKeyframes
 

LayoutTests/imported/w3c/web-platform-tests/css/css-animations/animation-important-001-expected.txt

@@ -1,9 +1,9 @@
 
 PASS Interpolated value is observable
-FAIL Important rules override animations assert_equals: expected "rgb(0, 128, 0)" but got "rgb(15, 15, 15)"
-FAIL Non-overriden interpolations are observable assert_equals: expected "rgb(0, 128, 0)" but got "rgb(15, 15, 15)"
+PASS Important rules override animations
+PASS Non-overriden interpolations are observable
 FAIL Important rules override animations (::before) assert_equals: expected "rgb(0, 128, 0)" but got "rgb(15, 15, 15)"
 PASS Important rules do not override animations on :visited as seen from JS
-FAIL Standard property animations appearing via setKeyframes do not override important declarations assert_equals: expected "rgb(255, 255, 255)" but got "rgb(15, 15, 15)"
-FAIL Custom property animations appearing via setKeyframes do not override important declarations assert_equals: expected "10px" but got "150px"
+PASS Standard property animations appearing via setKeyframes do not override important declarations
+PASS Custom property animations appearing via setKeyframes do not override important declarations
 

LayoutTests/imported/w3c/web-platform-tests/css/css-animations/keyframes-unrelated-custom-property-expected.txt

@@ -1,3 +1,3 @@
 
-FAIL Unrelated custom properties do not conflict with each other assert_equals: expected "rgb(0, 128, 0)" but got "rgb(255, 0, 0)"
+PASS Unrelated custom properties do not conflict with each other
 

LayoutTests/imported/w3c/web-platform-tests/css/css-properties-values-api/animation/custom-property-animation-non-inherited-used-by-standard-property-expected.txt

@@ -1,3 +1,3 @@
 
-FAIL Animating a non-inherited CSS variable is reflected on a standard property using that variable as a value assert_equals: expected "25px" but got "0px"
+PASS Animating a non-inherited CSS variable is reflected on a standard property using that variable as a value
 

LayoutTests/imported/w3c/web-platform-tests/css/css-properties-values-api/animation/custom-property-transition-non-inherited-used-by-standard-property-expected.txt

@@ -1,3 +1,3 @@
 
-FAIL Running a transition a non-inherited CSS variable is reflected on a standard property using that variable as a value assert_equals: expected "150px" but got "200px"
+PASS Running a transition a non-inherited CSS variable is reflected on a standard property using that variable as a value
 

LayoutTests/imported/w3c/web-platform-tests/css/css-transitions/transition-base-response-001-expected.txt

@@ -1,5 +1,5 @@
 
-FAIL em units respond to font-size transition assert_equals: expected "15px" but got "20px"
-FAIL ex units respond to font-size transition assert_equals: expected "6.71875px" but got "8.96875px"
+PASS em units respond to font-size transition
+PASS ex units respond to font-size transition
 PASS var() references respond to custom property transition
 

LayoutTests/imported/w3c/web-platform-tests/css/css-transitions/transition-base-response-003-expected.txt

@@ -1,3 +1,3 @@
 
-FAIL Identical elements are all responsive to font-size transition assert_equals: expected "15px" but got "20px"
+PASS Identical elements are all responsive to font-size transition
 

LayoutTests/imported/w3c/web-platform-tests/css/css-variables/variable-animation-from-to-expected.txt

@@ -1,7 +1,7 @@
 This text should animate from blue to green over a period of 1 second.
 
 PASS Verify CSS variable value before animation
-FAIL Verify substituted color value before animation assert_equals: color is blue before animation runs expected "rgb(0, 0, 255)" but got "rgb(255, 0, 0)"
+PASS Verify substituted color value before animation
 PASS Verify CSS variable value after animation
-FAIL Verify substituted color value after animation assert_equals: color is green after animation runs expected "rgb(0, 128, 0)" but got "rgb(255, 0, 0)"
+PASS Verify substituted color value after animation
 

LayoutTests/imported/w3c/web-platform-tests/css/css-variables/variable-animation-over-transition-expected.txt

@@ -1,7 +1,7 @@
 This text should animate from blue to green over a period of 1 second. The transition is not visible because the animation overrides it.
 
 PASS Verify CSS variable value before animation
-FAIL Verify substituted color value before animation assert_equals: color is blue before animation runs expected "rgb(0, 0, 255)" but got "rgb(255, 0, 0)"
+PASS Verify substituted color value before animation
 PASS Verify CSS variable value after animation
-FAIL Verify substituted color value after animation assert_equals: color is green after animation runs expected "rgb(0, 128, 0)" but got "rgb(0, 0, 0)"
+PASS Verify substituted color value after animation
 

Source/WebCore/animation/KeyframeEffectStack.cpp

@@ -140,7 +140,7 @@ void KeyframeEffectStack::setCSSAnimationList(RefPtr<const AnimationList>&& cssA
     m_isSorted = false;
 }
 
-OptionSet<AnimationImpact> KeyframeEffectStack::applyKeyframeEffects(RenderStyle& targetStyle, const RenderStyle* previousLastStyleChangeEventStyle, const Style::ResolutionContext& resolutionContext)
+OptionSet<AnimationImpact> KeyframeEffectStack::applyKeyframeEffects(RenderStyle& targetStyle, HashSet<AnimatableProperty>& affectedProperties, const RenderStyle* previousLastStyleChangeEventStyle, const Style::ResolutionContext& resolutionContext)
 {
     OptionSet<AnimationImpact> impact;
 
@@ -174,13 +174,8 @@ OptionSet<AnimationImpact> KeyframeEffectStack::applyKeyframeEffects(RenderStyle
         };
 
         auto cssVariableChanged = [&]() {
-            auto customPropertyValueMapDidChange = [](const CustomPropertyValueMap& a, const CustomPropertyValueMap& b) {
-                return &a != &b && a != b;
-            };
-
             if (previousLastStyleChangeEventStyle && effect->containsCSSVariableReferences()) {
-                if (customPropertyValueMapDidChange(previousLastStyleChangeEventStyle->inheritedCustomProperties(), targetStyle.inheritedCustomProperties())
-                    || customPropertyValueMapDidChange(previousLastStyleChangeEventStyle->nonInheritedCustomProperties(), targetStyle.nonInheritedCustomProperties()))
+                if (!previousLastStyleChangeEventStyle->customPropertiesEqual(targetStyle))
                     return true;
             }
             return false;
@@ -206,6 +201,8 @@ OptionSet<AnimationImpact> KeyframeEffectStack::applyKeyframeEffects(RenderStyle
             ASSERT(animation->timeline());
             animation->timeline()->animationTimingDidChange(*animation);
         }
+
+        affectedProperties.formUnion(effect->animatedProperties());
     }
 
     return impact;

Source/WebCore/animation/KeyframeEffectStack.h

@@ -55,7 +55,7 @@ class KeyframeEffectStack {
     bool containsProperty(CSSPropertyID) const;
     bool isCurrentlyAffectingProperty(CSSPropertyID) const;
     bool requiresPseudoElement() const;
-    OptionSet<AnimationImpact> applyKeyframeEffects(RenderStyle& targetStyle, const RenderStyle* previousLastStyleChangeEventStyle, const Style::ResolutionContext&);
+    OptionSet<AnimationImpact> applyKeyframeEffects(RenderStyle& targetStyle, HashSet<AnimatableProperty>& affectedProperties, const RenderStyle* previousLastStyleChangeEventStyle, const Style::ResolutionContext&);
     bool hasEffectWithImplicitKeyframes() const;
 
     void effectAbilityToBeAcceleratedDidChange(const KeyframeEffect&);

Source/WebCore/animation/WebAnimationTypes.h

@@ -53,7 +53,7 @@ struct WebAnimationsMarkableDoubleTraits {
     }
 };
 
-enum class AnimationImpact {
+enum class AnimationImpact : uint8_t {
     RequiresRecomposite     = 1 << 0,
     ForcesStackingContext   = 1 << 1
 };

Source/WebCore/dom/Element.h

@@ -81,7 +81,7 @@ class Text;
 class UniqueElementData;
 class WebAnimation;
 
-enum class AnimationImpact;
+enum class AnimationImpact : uint8_t;
 enum class EventHandling : uint8_t;
 enum class EventProcessing : uint8_t;
 enum class FullscreenNavigationUI : uint8_t;

Source/WebCore/html/shadow/TextControlInnerElements.cpp

@@ -137,7 +137,7 @@ std::optional<Style::ElementStyle> TextControlInnerElement::resolveCustomStyle(c
         newStyle->setFlexBasis(Length { pixels, LengthType::Fixed });
     }
 
-    return { WTFMove(newStyle) };
+    return Style::ElementStyle { WTFMove(newStyle) };
 }
 
 // MARK: TextControlInnerTextElement
@@ -198,7 +198,7 @@ RenderTextControlInnerBlock* TextControlInnerTextElement::renderer() const
 std::optional<Style::ElementStyle> TextControlInnerTextElement::resolveCustomStyle(const Style::ResolutionContext&, const RenderStyle* shadowHostStyle)
 {
     auto style = downcast<HTMLTextFormControlElement>(*shadowHost()).createInnerTextStyle(*shadowHostStyle);
-    return { makeUnique<RenderStyle>(WTFMove(style)) };
+    return Style::ElementStyle { makeUnique<RenderStyle>(WTFMove(style)) };
 }
 
 // MARK: TextControlPlaceholderElement

Source/WebCore/rendering/style/RenderStyle.cpp

@@ -2669,6 +2669,12 @@ void RenderStyle::setNonInheritedCustomPropertyValue(const AtomString& name, Ref
     m_rareNonInheritedData.access().customProperties.access().setCustomPropertyValue(name, WTFMove(value));
 }
 
+bool RenderStyle::customPropertiesEqual(const RenderStyle& other) const
+{
+    return m_rareNonInheritedData->customProperties == other.m_rareNonInheritedData->customProperties
+        && m_rareInheritedData->customProperties == other.m_rareInheritedData->customProperties;
+}
+
 const LengthBox& RenderStyle::scrollMargin() const
 {
     return m_rareNonInheritedData->scrollMargin;

Source/WebCore/rendering/style/RenderStyle.h

@@ -198,6 +198,7 @@ class RenderStyle {
     const CSSCustomPropertyValue* getCustomProperty(const AtomString&) const;
     void setInheritedCustomPropertyValue(const AtomString& name, Ref<CSSCustomPropertyValue>&&);
     void setNonInheritedCustomPropertyValue(const AtomString& name, Ref<CSSCustomPropertyValue>&&);
+    bool customPropertiesEqual(const RenderStyle&) const;
 
     void setUsesViewportUnits() { m_nonInheritedFlags.usesViewportUnits = true; }
     bool usesViewportUnits() const { return m_nonInheritedFlags.usesViewportUnits; }

Source/WebCore/style/ElementRuleCollector.cpp

@@ -92,6 +92,7 @@ ElementRuleCollector::ElementRuleCollector(const Element& element, const ScopeRu
     , m_userStyle(ruleSets.userStyle())
     , m_userAgentMediaQueryStyle(ruleSets.userAgentMediaQueryStyle())
     , m_selectorMatchingState(selectorMatchingState)
+    , m_result(makeUnique<MatchResult>())
 {
     ASSERT(!m_selectorMatchingState || m_selectorMatchingState->selectorFilter.parentStackIsConsistent(element.parentNode()));
 }
@@ -100,14 +101,20 @@ ElementRuleCollector::ElementRuleCollector(const Element& element, const RuleSet
     : m_element(element)
     , m_authorStyle(authorStyle)
     , m_selectorMatchingState(selectorMatchingState)
+    , m_result(makeUnique<MatchResult>())
 {
     ASSERT(!m_selectorMatchingState || m_selectorMatchingState->selectorFilter.parentStackIsConsistent(element.parentNode()));
 }
 
 const MatchResult& ElementRuleCollector::matchResult() const
 {
     ASSERT(m_mode == SelectorChecker::Mode::ResolvingStyle);
-    return m_result;
+    return *m_result;
+}
+
+std::unique_ptr<MatchResult> ElementRuleCollector::releaseMatchResult()
+{
+    return WTFMove(m_result);
 }
 
 const Vector<RefPtr<const StyleRule>>& ElementRuleCollector::matchedRuleList() const
@@ -134,7 +141,7 @@ inline void ElementRuleCollector::addElementStyleProperties(const StylePropertie
         return;
 
     if (!isCacheable)
-        m_result.isCacheable = false;
+        m_result->isCacheable = false;
 
     auto matchedProperty = MatchedProperties { propertySet };
     matchedProperty.cascadeLayerPriority = priority;
@@ -214,15 +221,15 @@ void ElementRuleCollector::collectMatchingRules(const MatchRequest& matchRequest
 }
 
 
-Vector<MatchedProperties>& ElementRuleCollector::declarationsForOrigin(MatchResult& matchResult, DeclarationOrigin declarationOrigin)
+Vector<MatchedProperties>& ElementRuleCollector::declarationsForOrigin(DeclarationOrigin declarationOrigin)
 {
     switch (declarationOrigin) {
-    case DeclarationOrigin::UserAgent: return matchResult.userAgentDeclarations;
-    case DeclarationOrigin::User: return matchResult.userDeclarations;
-    case DeclarationOrigin::Author: return matchResult.authorDeclarations;
+    case DeclarationOrigin::UserAgent: return m_result->userAgentDeclarations;
+    case DeclarationOrigin::User: return m_result->userDeclarations;
+    case DeclarationOrigin::Author: return m_result->authorDeclarations;
     }
     ASSERT_NOT_REACHED();
-    return matchResult.authorDeclarations;
+    return m_result->authorDeclarations;
 }
 
 void ElementRuleCollector::sortAndTransferMatchedRules(DeclarationOrigin declarationOrigin)
@@ -238,7 +245,7 @@ void ElementRuleCollector::sortAndTransferMatchedRules(DeclarationOrigin declara
 void ElementRuleCollector::transferMatchedRules(DeclarationOrigin declarationOrigin, std::optional<ScopeOrdinal> fromScope)
 {
     if (m_mode != SelectorChecker::Mode::CollectingRules)
-        declarationsForOrigin(m_result, declarationOrigin).reserveCapacity(m_matchedRules.size());
+        declarationsForOrigin(declarationOrigin).reserveCapacity(m_matchedRules.size());
 
     for (; m_matchedRuleTransferIndex < m_matchedRules.size(); ++m_matchedRuleTransferIndex) {
         auto& matchedRule = m_matchedRules[m_matchedRuleTransferIndex];
@@ -649,7 +656,7 @@ void ElementRuleCollector::addMatchedProperties(MatchedProperties&& matchedPrope
 {
     // FIXME: This should be moved to the matched properties cache code.
     auto computeIsCacheable = [&] {
-        if (!m_result.isCacheable)
+        if (!m_result->isCacheable)
             return false;
 
         if (matchedProperties.styleScopeOrdinal != ScopeOrdinal::Element)
@@ -683,15 +690,15 @@ void ElementRuleCollector::addMatchedProperties(MatchedProperties&& matchedPrope
         return true;
     };
 
-    m_result.isCacheable = computeIsCacheable();
+    m_result->isCacheable = computeIsCacheable();
 
-    declarationsForOrigin(m_result, declarationOrigin).append(WTFMove(matchedProperties));
+    declarationsForOrigin(declarationOrigin).append(WTFMove(matchedProperties));
 }
 
 void ElementRuleCollector::addAuthorKeyframeRules(const StyleRuleKeyframe& keyframe)
 {
-    ASSERT(m_result.authorDeclarations.isEmpty());
-    m_result.authorDeclarations.append({ &keyframe.properties(), SelectorChecker::MatchAll, propertyAllowlistForPseudoId(m_pseudoElementRequest.pseudoId) });
+    ASSERT(m_result->authorDeclarations.isEmpty());
+    m_result->authorDeclarations.append({ &keyframe.properties(), SelectorChecker::MatchAll, propertyAllowlistForPseudoId(m_pseudoElementRequest.pseudoId) });
 }
 
 }

Source/WebCore/style/ElementRuleCollector.h

@@ -86,6 +86,8 @@ class ElementRuleCollector {
     bool hasAnyMatchingRules(const RuleSet&);
 
     const MatchResult& matchResult() const;
+    std::unique_ptr<MatchResult> releaseMatchResult();
+
     const Vector<RefPtr<const StyleRule>>& matchedRuleList() const;
 
     void clearMatchedRules();
@@ -120,7 +122,7 @@ class ElementRuleCollector {
     void sortMatchedRules();
 
     enum class DeclarationOrigin { UserAgent, User, Author };
-    static Vector<MatchedProperties>& declarationsForOrigin(MatchResult&, DeclarationOrigin);
+    Vector<MatchedProperties>& declarationsForOrigin(DeclarationOrigin);
     void sortAndTransferMatchedRules(DeclarationOrigin);
     void transferMatchedRules(DeclarationOrigin, std::optional<ScopeOrdinal> forScope = { });
 
@@ -146,7 +148,7 @@ class ElementRuleCollector {
     // Output.
     Vector<RefPtr<const StyleRule>> m_matchedRuleList;
     bool m_didMatchUncommonAttributeSelector { false };
-    MatchResult m_result;
+    std::unique_ptr<MatchResult> m_result;
     Relations m_styleRelations;
     PseudoIdSet m_matchedPseudoElementIds;
 };

Source/WebCore/style/MatchResult.h

@@ -45,6 +45,8 @@ struct MatchedProperties {
 };
 
 struct MatchResult {
+    WTF_MAKE_STRUCT_FAST_ALLOCATED;
+
     bool isCacheable { true };
     Vector<MatchedProperties> userAgentDeclarations;
     Vector<MatchedProperties> userDeclarations;