[css-nesting] Implement new cascade behavior for mixed declarations a…

…nd rules

https://bugs.webkit.org/show_bug.cgi?id=275365
rdar://130094168

Reviewed by NOBODY (OOPS!).

Previously, any bare declaration inside a style rule, whatever its position
relative to other rules, would be shifted up at the top of the rule
to be able to represent a style rule with a single continous leading block of declarations.

This behavior has been fixed so the order of the declarations is respected
during cascade. w3c/csswg-drafts#10234

This introduces a new CSSNestedDeclarations object (a new kind of CSS style rule) to be able to store those block of declarations in-between rules and fit with the already existing RuleData/RuleSet mechanism. It is purposely not exposed as a style rule in the CSSOM.

The CSSOM insertRule() functions (on CSSStyleRule and on CSSGroupingRule) is modified to allow inserting block of declarations. w3c/csswg-drafts#10520

https://drafts.csswg.org/css-nesting/#the-cssnestrule

* LayoutTests/TestExpectations:
* LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/cssom-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/cssom.html:
* LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/mixed-declarations-rules-expected.html: Added.
* LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/mixed-declarations-rules.html: Added.
* LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/nested-declarations-cssom-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/nested-declarations-matching-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/serialize-group-rules-with-decls-expected.txt:
* LayoutTests/imported/w3c/web-platform-tests/css/css-syntax/custom-property-rule-ambiguity.html:

  Manual sync from WPT

* Source/WebCore/CMakeLists.txt:
* Source/WebCore/DerivedSources-input.xcfilelist:
* Source/WebCore/DerivedSources-output.xcfilelist:
* Source/WebCore/DerivedSources.make:
* Source/WebCore/Sources.txt:
* Source/WebCore/WebCore.xcodeproj/project.pbxproj:
* Source/WebCore/bindings/js/JSCSSRuleCustom.cpp:
(WebCore::toJSNewlyCreated):
* Source/WebCore/css/CSSGradientValue.h:
* Source/WebCore/css/CSSNestedDeclarations.cpp: Added.
(WebCore::CSSNestedDeclarations::CSSNestedDeclarations):
(WebCore::CSSNestedDeclarations::style):
(WebCore::CSSNestedDeclarations::cssText const):
(WebCore::CSSNestedDeclarations::reattach):
* Source/WebCore/css/CSSNestedDeclarations.h: Added.
* Source/WebCore/css/CSSNestedDeclarations.idl: Added.
* Source/WebCore/css/CSSShapeSegmentValue.cpp:
* Source/WebCore/css/CSSStyleRule.cpp:
(WebCore::CSSStyleRule::insertRule):
* Source/WebCore/css/CSSStyleRule.h:
* Source/WebCore/css/StyleRule.cpp:
(WebCore::StyleRuleBase::visitDerived):
(WebCore::StyleRuleBase::createCSSOMWrapper const):
(WebCore::StyleRuleNestedDeclarations::StyleRuleNestedDeclarations):
(WebCore::StyleRuleNestedDeclarations::debugDescription const):
* Source/WebCore/css/StyleRule.h:
(WebCore::StyleRuleBase::isStyleRuleNestedDeclarations const):
(isType):
* Source/WebCore/css/StyleRuleType.h:
* Source/WebCore/css/StyleSheetContents.cpp:
(WebCore::StyleSheetContents::traverseRules const):
(WebCore::StyleSheetContents::traverseSubresources const):
(WebCore::StyleSheetContents::mayDependOnBaseURL const):
* Source/WebCore/css/calc/CSSCalcTree+Simplification.cpp:
* Source/WebCore/css/calc/CSSCalcTree+Simplification.h:
* Source/WebCore/css/parser/CSSParser.h:
* Source/WebCore/css/parser/CSSParserImpl.cpp:
(WebCore::CSSParserImpl::parseNestedDeclarations):
(WebCore::CSSParserImpl::createNestedDeclarationsRule):
(WebCore::CSSParserImpl::consumeNestedGroupRules):
(WebCore::CSSParserImpl::consumeBlockContent):
(WebCore::CSSParserImpl::createNestingParentRule): Deleted.
* Source/WebCore/css/parser/CSSParserImpl.h:
* Source/WebCore/css/parser/CSSPropertyParserConsumer+Image.cpp:
(WebCore::CSSPropertyParserHelpers::consumeImageSetResolutionOrTypeFunction):
* Source/WebCore/css/parser/CSSPropertyParserConsumer+Image.h:
* Source/WebCore/inspector/InspectorStyleSheet.cpp:
(WebCore::flatteningStrategyForStyleRuleType):
* Source/WebCore/style/RuleSetBuilder.cpp:
(WebCore::Style::RuleSetBuilder::addChildRule):
(WebCore::Style::RuleSetBuilder::addStyleRule):
* Source/WebCore/style/RuleSetBuilder.h:

LayoutTests/TestExpectations

@@ -8068,10 +8068,6 @@ imported/w3c/web-platform-tests/css/css-anchor-position/anchor-scroll-js-expose.
 
 fast/canvas/image-buffer-resource-limits.html [ Slow ]
 
-# CSS Nesting interleaved declarations and rules
-# https://bugs.webkit.org/show_bug.cgi?id=275365
-imported/w3c/web-platform-tests/css/css-nesting/nesting-basic.html [ ImageOnlyFailure ]
-
 # Standardized CSS zoom tests.
 imported/w3c/web-platform-tests/css/css-viewport/width.html [ ImageOnlyFailure ]
 imported/w3c/web-platform-tests/css/css-viewport/zoom/border-spacing.html [ ImageOnlyFailure ]
@@ -8091,6 +8087,9 @@ imported/w3c/web-platform-tests/css/css-viewport/zoom/word-spacing.html [ ImageO
 # WebRTC Encoded Transform - Test Expectation - Crashes on 'mac-wk2' debug and gtk-wk2 / wpe-wk2
 webkit.org/b/275663 imported/w3c/web-platform-tests/webrtc-encoded-transform/script-transform-generateKeyFrame-simulcast.https.html [ Skip ]
 
+# https://bugs.webkit.org/show_bug.cgi?id=276663
+inspector/css/getMatchedStylesForNodeNestingStyleGrouping.html [ Failure ]
+
 # https://bugs.webkit.org/show_bug.cgi?id=266843 flakey tests with racey promise console.logs
 imported/w3c/web-platform-tests/dom/observable/tentative/observable-from.any.html [ Skip ]
 

LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/cssom-expected.txt

@@ -16,4 +16,5 @@ PASS Simple CSSOM manipulation of subrules 8
 PASS Simple CSSOM manipulation of subrules 9
 PASS Simple CSSOM manipulation of subrules 10
 PASS Mutating the selectorText of outer rule invalidates inner rules
+PASS Manipulation of nested declarations through CSSOM
 

LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/cssom.html

@@ -185,4 +185,41 @@
     assert_equals(getComputedStyle(inner1).zIndex, '1');
     assert_equals(getComputedStyle(inner2).zIndex, '1');
   }, 'Mutating the selectorText of outer rule invalidates inner rules');
+
+  // CSSNestedDeclarations
+  test((t) => {
+    const main = document.createElement('main');
+    main.innerHTML = `
+      <style id="main_ss">
+        div {
+          z-index: 1;
+          &.test { foo:bar; }
+        }
+      </style>
+      <div id="outer" class="test">
+      </div>
+    `;
+    document.documentElement.append(main);
+    t.add_cleanup(() => main.remove());
+    assert_equals(getComputedStyle(outer).zIndex, '1');
+    const main_ss = document.getElementById("main_ss").sheet;
+    const rule = main_ss.cssRules[0];
+    assert_equals(rule.cssRules.length, 1);
+    rule.insertRule('z-index: 3;');
+    assert_equals(rule.cssRules.length, 2);
+    assert_equals(getComputedStyle(outer).zIndex, '3');
+
+    // Throw only when no valid declaration  https://github.com/w3c/csswg-drafts/issues/10520
+    assert_throws_dom('SyntaxError', () => { rule.insertRule('nothing-to-insert-because-invalid-property-should-throw: 2;'); });
+    assert_equals(rule.cssRules.length, 2);
+
+    // Test the insertion of nested declarations inside grouping rule
+    rule.insertRule('@media screen { a { color: blue; }}',2);
+    assert_equals(rule.cssRules.length, 3);
+    const mediaRule = rule.cssRules[2];
+    mediaRule.insertRule('z-index: 3;');
+    assert_equals(mediaRule.cssRules.length, 2);
+    assert_throws_dom('SyntaxError', () => { mediaRule.insertRule('nothing-to-insert-because-invalid-property-should-throw: 2;'); });
+  }, 'Manipulation of nested declarations through CSSOM');
+
 </script>

LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/mixed-declarations-rules-expected.html

@@ -0,0 +1,18 @@
+<!DOCTYPE html>
+<title>Conditional rules with nesting</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting-1/">
+<style>
+  .test {
+    background-color: green;
+    width: 100px;
+    height: 100px;
+  }
+
+  body * + * {
+    margin-top: 8px;
+  }
+</style>
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test"><div></div></div>
+</body>

LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/mixed-declarations-rules.html

@@ -0,0 +1,20 @@
+<!DOCTYPE html>
+<title>Mixed declarations and rules</title>
+<link rel="help" href="https://drafts.csswg.org/css-nesting/#nested-declarations-rule">
+
+<style>
+div {
+  width: 100px;
+  height: 100px;
+  background-color: blue;
+  @media all {
+    background-color: red;
+  }
+  background-color: green;
+}
+</style>
+
+<body>
+  <p>Tests pass if <strong>block is green</strong></p>
+  <div class="test"></div>
+</body>

LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/nested-declarations-cssom-expected.txt

@@ -1,12 +1,12 @@
 
-FAIL Trailing declarations assert_equals: expected 2 but got 1
-FAIL Mixed declarations assert_equals: expected 6 but got 3
-FAIL CSSNestedDeclarations.style assert_equals: expected 2 but got 1
-FAIL Nested group rule assert_equals: expected 2 but got 1
-FAIL Nested @scope rule assert_equals: expected 2 but got 1
-FAIL Inner rule starting with an ident assert_equals: expected 4 but got 2
-FAIL Inserting a CSSNestedDeclaration rule into style rule The string did not match the expected pattern.
-FAIL Inserting a CSSNestedDeclaration rule into nested group rule The string did not match the expected pattern.
+PASS Trailing declarations
+PASS Mixed declarations
+PASS CSSNestedDeclarations.style
+PASS Nested group rule
+PASS Nested @scope rule
+PASS Inner rule starting with an ident
+PASS Inserting a CSSNestedDeclaration rule into style rule
+PASS Inserting a CSSNestedDeclaration rule into nested group rule
 PASS Attempting to insert a CSSNestedDeclaration rule into top-level @media rule
 PASS Attempting to insert a CSSNestedDeclaration rule into a stylesheet
 PASS Attempting to insert a CSSNestedDeclaration rule, empty block

LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/nested-declarations-matching-expected.txt

@@ -1,14 +1,14 @@
 A1
 A2
 
-FAIL Trailing declarations apply after any preceding rules assert_equals: expected "PASS" but got "FAIL"
-FAIL Trailing declarations apply after any preceding rules (no leading) assert_equals: expected "PASS" but got "FAIL"
-FAIL Trailing declarations apply after any preceding rules (multiple) assert_equals: expected "PASS" but got "FAIL"
+PASS Trailing declarations apply after any preceding rules
+PASS Trailing declarations apply after any preceding rules (no leading)
+PASS Trailing declarations apply after any preceding rules (multiple)
 PASS Nested declarations rule has same specificity as outer selector
 PASS Nested declarations rule has top-level specificity behavior
-FAIL Nested declarations rule has top-level specificity behavior (max matching) assert_equals: expected "PASS" but got "FAIL"
-FAIL Bare declartaion in nested grouping rule can match pseudo-element assert_equals: expected "PASS" but got "FAIL"
-FAIL Nested group rules have top-level specificity behavior assert_equals: expected "PASS" but got "FAIL"
+PASS Nested declarations rule has top-level specificity behavior (max matching)
+PASS Bare declartaion in nested grouping rule can match pseudo-element
+PASS Nested group rules have top-level specificity behavior
 FAIL Nested @scope rules behave like :where(:scope) assert_equals: expected "PASS" but got "FAIL"
 FAIL Nested @scope rules behave like :where(:scope) (trailing) assert_equals: expected "PASS" but got "FAIL"
 FAIL Nested declarations rule responds to parent selector text change assert_equals: expected "rgb(0, 128, 0)" but got "rgb(255, 0, 0)"

LayoutTests/imported/w3c/web-platform-tests/css/css-nesting/serialize-group-rules-with-decls-expected.txt

@@ -1,13 +1,13 @@
 
 PASS Declarations are serialized on one line, rules on two.
-FAIL Mixed declarations/rules are on two lines. assert_equals: Mixed declarations/rules are on two lines. expected "div {\n  @media screen {\n  color: red; background-color: green;\n}\n}" but got "div {\n  @media screen {\n  & { color: red; background-color: green; }\n}\n}"
-FAIL Implicit rule is serialized assert_equals: Implicit rule is serialized expected "div {\n  @supports selector(&) {\n  color: red; background-color: green;\n}\n  &:hover { color: navy; }\n}" but got "div {\n  @supports selector(&) {\n  & { color: red; background-color: green; }\n}\n  &:hover { color: navy; }\n}"
+PASS Mixed declarations/rules are on two lines.
+PASS Implicit rule is serialized
 PASS Implicit rule not removed
 PASS Implicit + empty hover rule
 PASS Implicit like rule not in first position
 PASS Two implicit-like rules
-FAIL Implicit like rule after decls assert_equals: Implicit like rule after decls expected "div {\n  @media screen {\n  color: red;\n  & { color: red; }\n}\n}" but got "div {\n  @media screen {\n  & { color: red; }\n  & { color: red; }\n}\n}"
-FAIL Implicit like rule after decls, missing closing braces assert_equals: Implicit like rule after decls, missing closing braces expected "div {\n  @media screen {\n  color: red;\n  & { color: blue; }\n}\n}" but got "div {\n  @media screen {\n  & { color: red; }\n  & { color: blue; }\n}\n}"
+PASS Implicit like rule after decls
+PASS Implicit like rule after decls, missing closing braces
 PASS Implicit like rule with other selectors
 PASS Implicit-like rule in style rule
 PASS Empty conditional rule

LayoutTests/imported/w3c/web-platform-tests/css/css-syntax/custom-property-rule-ambiguity.html

@@ -46,11 +46,12 @@
     assert_equals(rules.length, 1);
     assert_equals(rules[0].selectorText, 'div');
     let div = rules[0];
-    let x = div.style.getPropertyValue('--x');
-    assert_equals(x.trim(), 'hover { }\n    .b { }');
     let childRules = div.cssRules;
-    assert_equals(childRules.length, 1);
+    assert_equals(childRules.length, 2);
     assert_equals(childRules[0].selectorText, '& .a');
+    assert_true(childRules[1] instanceof CSSNestedDeclarations)
+    let x = childRules[1].style.getPropertyValue('--x');
+    assert_equals(x.trim(), 'hover { }\n    .b { }');
   }, 'Nested rule that looks like a custom property declaration');
 </script>
 

Source/WebCore/CMakeLists.txt

@@ -929,6 +929,7 @@ set(WebCore_NON_SVG_IDL_FILES
     css/CSSLayerStatementRule.idl
     css/CSSMediaRule.idl
     css/CSSNamespaceRule.idl
+    css/CSSNestedDeclarations.idl
     css/CSSPaintCallback.idl
     css/CSSPaintSize.idl
     css/CSSPageRule.idl

Source/WebCore/DerivedSources-input.xcfilelist

@@ -1243,6 +1243,7 @@ $(PROJECT_DIR)/css/CSSLayerBlockRule.idl
 $(PROJECT_DIR)/css/CSSLayerStatementRule.idl
 $(PROJECT_DIR)/css/CSSMediaRule.idl
 $(PROJECT_DIR)/css/CSSNamespaceRule.idl
+$(PROJECT_DIR)/css/CSSNestedDeclarations.idl
 $(PROJECT_DIR)/css/CSSPageRule.idl
 $(PROJECT_DIR)/css/CSSPaintCallback.idl
 $(PROJECT_DIR)/css/CSSPaintSize.idl

Source/WebCore/DerivedSources-output.xcfilelist

@@ -433,6 +433,8 @@ $(BUILT_PRODUCTS_DIR)/DerivedSources/WebCore/JSCSSMediaRule.cpp
 $(BUILT_PRODUCTS_DIR)/DerivedSources/WebCore/JSCSSMediaRule.h
 $(BUILT_PRODUCTS_DIR)/DerivedSources/WebCore/JSCSSNamespaceRule.cpp
 $(BUILT_PRODUCTS_DIR)/DerivedSources/WebCore/JSCSSNamespaceRule.h
+$(BUILT_PRODUCTS_DIR)/DerivedSources/WebCore/JSCSSNestedDeclarations.cpp
+$(BUILT_PRODUCTS_DIR)/DerivedSources/WebCore/JSCSSNestedDeclarations.h
 $(BUILT_PRODUCTS_DIR)/DerivedSources/WebCore/JSCSSNumericArray.cpp
 $(BUILT_PRODUCTS_DIR)/DerivedSources/WebCore/JSCSSNumericArray.h
 $(BUILT_PRODUCTS_DIR)/DerivedSources/WebCore/JSCSSNumericBaseType.cpp

Source/WebCore/DerivedSources.make

@@ -964,6 +964,7 @@ JS_BINDING_IDLS := \
     $(WebCore)/css/CSSKeyframesRule.idl \
     $(WebCore)/css/CSSMediaRule.idl \
     $(WebCore)/css/CSSNamespaceRule.idl \
+    $(WebCore)/css/CSSNestedDeclarations.idl \
     $(WebCore)/css/CSSPageRule.idl \
     $(WebCore)/css/CSSPaintCallback.idl \
     $(WebCore)/css/CSSPaintSize.idl \

Source/WebCore/Sources.txt

@@ -897,6 +897,7 @@ css/CSSMarkup.cpp
 css/CSSMediaRule.cpp
 css/CSSNamedImageValue.cpp
 css/CSSNamespaceRule.cpp
+css/CSSNestedDeclarations.cpp
 css/CSSOffsetRotateValue.cpp
 css/CSSPageRule.cpp
 css/CSSPaintImageValue.cpp
@@ -3417,6 +3418,7 @@ JSCSSLayerBlockRule.cpp
 JSCSSLayerStatementRule.cpp
 JSCSSMediaRule.cpp
 JSCSSNamespaceRule.cpp
+JSCSSNestedDeclarations.cpp
 JSCSSPageRule.cpp
 JSCSSPaintCallback.cpp
 JSCSSPaintSize.cpp

Source/WebCore/WebCore.xcodeproj/project.pbxproj

@@ -4977,6 +4977,7 @@
 		BE913D80181EF92400DCB09E /* TrackPrivateBase.h in Headers */ = {isa = PBXBuildFile; fileRef = BE913D7F181EF8E500DCB09E /* TrackPrivateBase.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		BE961C5518AD338C00D07DC5 /* InbandDataTextTrack.h in Headers */ = {isa = PBXBuildFile; fileRef = BE961C5318AD337C00D07DC5 /* InbandDataTextTrack.h */; };
 		BEA807C90F714A0300524199 /* SelectionGeometry.h in Headers */ = {isa = PBXBuildFile; fileRef = BEA807C70F714A0300524199 /* SelectionGeometry.h */; settings = {ATTRIBUTES = (Private, ); }; };
+		BEAA24022C34A9E600C37BBE /* CSSNestedDeclarations.h in Headers */ = {isa = PBXBuildFile; fileRef = BEAA23FF2C34A63900C37BBE /* CSSNestedDeclarations.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		BEF29EEB1715DD0900C4B4C9 /* AudioTrackPrivate.h in Headers */ = {isa = PBXBuildFile; fileRef = BEF29EE91715DD0900C4B4C9 /* AudioTrackPrivate.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		BEF29EEC1715DD0900C4B4C9 /* VideoTrackPrivate.h in Headers */ = {isa = PBXBuildFile; fileRef = BEF29EEA1715DD0900C4B4C9 /* VideoTrackPrivate.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		C0C054CB1118C8E400CE2636 /* CodeGenerator.pm in Headers */ = {isa = PBXBuildFile; fileRef = 93F8B3050A300FE100F61AB8 /* CodeGenerator.pm */; settings = {ATTRIBUTES = (Private, ); }; };
@@ -18132,6 +18133,9 @@
 		BE983D95052A2E0A00892D85 /* WebCoreKeyboardUIMode.h */ = {isa = PBXFileReference; fileEncoding = 30; indentWidth = 4; lastKnownFileType = sourcecode.c.h; path = WebCoreKeyboardUIMode.h; sourceTree = "<group>"; tabWidth = 8; usesTabs = 0; };
 		BEA807C60F714A0300524199 /* SelectionGeometry.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = SelectionGeometry.cpp; sourceTree = "<group>"; };
 		BEA807C70F714A0300524199 /* SelectionGeometry.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = SelectionGeometry.h; sourceTree = "<group>"; };
+		BEAA23FF2C34A63900C37BBE /* CSSNestedDeclarations.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = CSSNestedDeclarations.h; sourceTree = "<group>"; };
+		BEAA24002C34A64400C37BBE /* CSSNestedDeclarations.idl */ = {isa = PBXFileReference; lastKnownFileType = text; path = CSSNestedDeclarations.idl; sourceTree = "<group>"; };
+		BEAA24012C34A94300C37BBE /* CSSNestedDeclarations.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = CSSNestedDeclarations.cpp; sourceTree = "<group>"; };
 		BED8BAF528EB355F003C7D65 /* CSSFontFeatureValuesRule.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = CSSFontFeatureValuesRule.cpp; sourceTree = "<group>"; };
 		BED8BAF628EB3560003C7D65 /* CSSFontFeatureValuesRule.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = CSSFontFeatureValuesRule.h; sourceTree = "<group>"; };
 		BEF29EE91715DD0900C4B4C9 /* AudioTrackPrivate.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = AudioTrackPrivate.h; sourceTree = "<group>"; };
@@ -36360,6 +36364,9 @@
 				94E8394E1DFB2700007BC6A7 /* CSSNamespaceRule.cpp */,
 				94E8394F1DFB2700007BC6A7 /* CSSNamespaceRule.h */,
 				94E839501DFB29A4007BC6A7 /* CSSNamespaceRule.idl */,
+				BEAA24012C34A94300C37BBE /* CSSNestedDeclarations.cpp */,
+				BEAA23FF2C34A63900C37BBE /* CSSNestedDeclarations.h */,
+				BEAA24002C34A64400C37BBE /* CSSNestedDeclarations.idl */,
 				668500B8273EEB7800FCCAD6 /* CSSOffsetRotateValue.cpp */,
 				668500B9273EEB7800FCCAD6 /* CSSOffsetRotateValue.h */,
 				A80E6CCB0A1989CA007FB8C5 /* CSSPageRule.cpp */,
@@ -39088,6 +39095,7 @@
 				A80E6D030A1989CA007FB8C5 /* CSSMediaRule.h in Headers */,
 				314BE3A11B30F6B700141982 /* CSSNamedImageValue.h in Headers */,
 				94E839511DFB2A0E007BC6A7 /* CSSNamespaceRule.h in Headers */,
+				BEAA24022C34A9E600C37BBE /* CSSNestedDeclarations.h in Headers */,
 				2AEF6FDB26E7ECC700326D02 /* CSSNumericArray.h in Headers */,
 				2AEF6FDC26E7ECCC00326D02 /* CSSNumericBaseType.h in Headers */,
 				2ACB4D2B26DC4ACE00BEB753 /* CSSNumericFactory.h in Headers */,

Source/WebCore/bindings/js/JSCSSRuleCustom.cpp

@@ -38,6 +38,7 @@
 #include "CSSLayerStatementRule.h"
 #include "CSSMediaRule.h"
 #include "CSSNamespaceRule.h"
+#include "CSSNestedDeclarations.h"
 #include "CSSPageRule.h"
 #include "CSSPropertyRule.h"
 #include "CSSScopeRule.h"
@@ -57,6 +58,7 @@
 #include "JSCSSLayerStatementRule.h"
 #include "JSCSSMediaRule.h"
 #include "JSCSSNamespaceRule.h"
+#include "JSCSSNestedDeclarations.h"
 #include "JSCSSPageRule.h"
 #include "JSCSSPropertyRule.h"
 #include "JSCSSScopeRule.h"
@@ -87,6 +89,8 @@ JSValue toJSNewlyCreated(JSGlobalObject*, JSDOMGlobalObject* globalObject, Ref<C
         return createWrapper<CSSStyleRule>(globalObject, WTFMove(rule));
     case StyleRuleType::StyleWithNesting:
         return createWrapper<CSSStyleRule>(globalObject, WTFMove(rule));
+    case StyleRuleType::NestedDeclarations:
+        return createWrapper<CSSNestedDeclarations>(globalObject, WTFMove(rule));
     case StyleRuleType::Media:
         return createWrapper<CSSMediaRule>(globalObject, WTFMove(rule));
     case StyleRuleType::FontFace:

Source/WebCore/css/CSSGradientValue.h

@@ -31,11 +31,10 @@
 #include "CSSPropertyParserConsumer+UnevaluatedCalc.h"
 #include "ColorInterpolationMethod.h"
 #include "Gradient.h"
+#include "StyleImage.h"
 
 namespace WebCore {
 
-class StyleImage;
-
 namespace Style {
 class BuilderState;
 }

Source/WebCore/css/CSSGroupingRule.cpp

@@ -32,6 +32,7 @@
 #include "CSSGroupingRule.h"
 
 #include "CSSParser.h"
+#include "CSSParserImpl.h"
 #include "CSSRuleList.h"
 #include "CSSStyleSheet.h"
 #include "StylePropertiesInlines.h"
@@ -69,13 +70,16 @@ ExceptionOr<unsigned> CSSGroupingRule::insertRule(const String& ruleString, unsi
     auto isNestedContext = hasStyleRuleAncestor() ? CSSParserEnum::IsNestedContext::Yes : CSSParserEnum::IsNestedContext::No;
     RefPtr<StyleRuleBase> newRule = CSSParser::parseRule(parserContext(), styleSheet ? &styleSheet->contents() : nullptr, ruleString, isNestedContext);
     if (!newRule) {
-        // SyntaxError: Raised if the specified rule has a syntax error and is unparsable.
-        return Exception { ExceptionCode::SyntaxError };
+        if (!hasStyleRuleAncestor())
+            return Exception { ExceptionCode::SyntaxError };
+        newRule = CSSParserImpl::parseNestedDeclarations(parserContext(), ruleString);
+        if (!newRule)
+            return Exception { ExceptionCode::SyntaxError };
     }
 
     if (newRule->isImportRule() || newRule->isNamespaceRule()) {
-        // FIXME: an HierarchyRequestError should also be thrown for a @charset or a nested
-        // @media rule. They are currently not getting parsed, resulting in a SyntaxError
+        // FIXME: an HierarchyRequestError should also be thrown for a @charset.
+        // They are currently not getting parsed, resulting in a SyntaxError
         // to get raised above.
 
         // HierarchyRequestError: Raised if the rule cannot be inserted at the specified
@@ -84,8 +88,7 @@ ExceptionOr<unsigned> CSSGroupingRule::insertRule(const String& ruleString, unsi
         return Exception { ExceptionCode::HierarchyRequestError };
     }
 
-    // Nesting inside style rule only accepts style rule or group rule
-    if (hasStyleRuleAncestor() && !newRule->isStyleRule() && !newRule->isGroupRule())
+    if (hasStyleRuleAncestor() && !newRule->isStyleRule() && !newRule->isGroupRule() && !newRule->isNestedDeclarationsRule())
         return Exception { ExceptionCode::HierarchyRequestError };
 
     CSSStyleSheet::RuleMutationScope mutationScope(this);