LibWeb/CSS: Parse nested declarations in insert_a_css_rule()

The spec algorithm changed at some point to support nested declarations,
but I only just noticed. The subtest regression is one we were passing
incorrectly.

Libraries/LibWeb/CSS/CSSGroupingRule.cpp

@@ -41,10 +41,14 @@ void CSSGroupingRule::clear_caches()
         rule->clear_caches();
 }
 
+// https://drafts.csswg.org/cssom/#dom-cssgroupingrule-insertrule
 WebIDL::ExceptionOr<u32> CSSGroupingRule::insert_rule(StringView rule, u32 index)
 {
-    TRY(m_rules->insert_a_css_rule(rule, index));
+    // The insertRule(rule, index) method must return the result of invoking insert a CSS rule rule into the child CSS
-    // NOTE: The spec doesn't say where to set the parent rule, so we'll do it here.
+    // rules at index, with the nested flag set.
+    TRY(m_rules->insert_a_css_rule(rule, index, CSSRuleList::Nested::Yes));
+
+    // AD-HOC: The spec doesn't say where to set the parent rule, so we'll do it here.
     m_rules->item(index)->set_parent_rule(this);
     return index;
 }

Libraries/LibWeb/CSS/CSSRuleList.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2024, Sam Atkins <sam@ladybird.org>
+ * Copyright (c) 2021-2025, Sam Atkins <sam@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -11,6 +11,7 @@
 #include <LibWeb/CSS/CSSKeyframesRule.h>
 #include <LibWeb/CSS/CSSLayerBlockRule.h>
 #include <LibWeb/CSS/CSSMediaRule.h>
+#include <LibWeb/CSS/CSSNestedDeclarations.h>
 #include <LibWeb/CSS/CSSRule.h>
 #include <LibWeb/CSS/CSSRuleList.h>
 #include <LibWeb/CSS/CSSSupportsRule.h>
@@ -52,8 +53,8 @@ void CSSRuleList::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_rules);
 }
 
-// https://www.w3.org/TR/cssom/#insert-a-css-rule
+// https://drafts.csswg.org/cssom/#insert-a-css-rule
-WebIDL::ExceptionOr<unsigned> CSSRuleList::insert_a_css_rule(Variant<StringView, CSSRule*> rule, u32 index)
+WebIDL::ExceptionOr<unsigned> CSSRuleList::insert_a_css_rule(Variant<StringView, CSSRule*> rule, u32 index, Nested nested)
 {
     // 1. Set length to the number of items in list.
     auto length = m_rules.size();
@@ -69,24 +70,37 @@ WebIDL::ExceptionOr<unsigned> CSSRuleList::insert_a_css_rule(Variant<StringView,
     CSSRule* new_rule = nullptr;
     if (rule.has<StringView>()) {
         new_rule = parse_css_rule(
-            CSS::Parser::ParsingParams { realm() },
+            Parser::ParsingParams { realm() },
             rule.get<StringView>());
     } else {
         new_rule = rule.get<CSSRule*>();
     }
 
-    // 4. If new rule is a syntax error, throw a SyntaxError exception.
+    // 4. If new rule is a syntax error, and nested is set, perform the following substeps:
+    if (!new_rule && nested == Nested::Yes) {
+        // - Set declarations to the results of performing parse a CSS declaration block, on argument rule.
+        auto declarations = parse_css_property_declaration_block(Parser::ParsingParams { realm() }, rule.get<StringView>());
+
+        // - If declarations is empty, throw a SyntaxError exception.
+        if (declarations.custom_properties.is_empty() && declarations.properties.is_empty())
+            return WebIDL::SyntaxError::create(realm(), "Unable to parse CSS declarations block."_string);
+
+        // - Otherwise, set new rule to a new nested declarations rule with declarations as it contents.
+        new_rule = CSSNestedDeclarations::create(realm(), CSSStyleProperties::create(realm(), move(declarations.properties), move(declarations.custom_properties)));
+    }
+
+    // 5. If new rule is a syntax error, throw a SyntaxError exception.
     if (!new_rule)
         return WebIDL::SyntaxError::create(realm(), "Unable to parse CSS rule."_string);
 
-    // FIXME: 5. If new rule cannot be inserted into list at the zero-index position index due to constraints specified by CSS, then throw a HierarchyRequestError exception. [CSS21]
+    // FIXME: 6. If new rule cannot be inserted into list at the zero-index position index due to constraints specified by CSS, then throw a HierarchyRequestError exception. [CSS21]
 
-    // FIXME: 6. If new rule is an @namespace at-rule, and list contains anything other than @import at-rules, and @namespace at-rules, throw an InvalidStateError exception.
+    // FIXME: 7. If new rule is an @namespace at-rule, and list contains anything other than @import at-rules, and @namespace at-rules, throw an InvalidStateError exception.
 
-    // 7. Insert new rule into list at the zero-indexed position index.
+    // 8. Insert new rule into list at the zero-indexed position index.
     m_rules.insert(index, *new_rule);
 
-    // 8. Return index.
+    // 9. Return index.
     if (on_change)
         on_change();
     return index;

Libraries/LibWeb/CSS/CSSRuleList.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2024, Sam Atkins <sam@ladybird.org>
+ * Copyright (c) 2021-2025, Sam Atkins <sam@ladybird.org>
  * Copyright (c) 2022, Andreas Kling <andreas@ladybird.org>
  * Copyright (c) 2023, Luke Wilde <lukew@serenityos.org>
  *
@@ -9,8 +9,6 @@
 #pragma once
 
 #include <AK/Function.h>
-#include <AK/Iterator.h>
-#include <AK/RefPtr.h>
 #include <LibWeb/Bindings/PlatformObject.h>
 #include <LibWeb/CSS/CSSRule.h>
 #include <LibWeb/Forward.h>
@@ -55,7 +53,11 @@ public:
     virtual Optional<JS::Value> item_value(size_t index) const override;
 
     WebIDL::ExceptionOr<void> remove_a_css_rule(u32 index);
-    WebIDL::ExceptionOr<unsigned> insert_a_css_rule(Variant<StringView, CSSRule*>, u32 index);
+    enum class Nested {
+        No,
+        Yes,
+    };
+    WebIDL::ExceptionOr<unsigned> insert_a_css_rule(Variant<StringView, CSSRule*>, u32 index, Nested = Nested::No);
 
     void for_each_effective_rule(TraversalOrder, Function<void(CSSRule const&)> const& callback) const;
     // Returns whether the match state of any media queries changed after evaluation.

Tests/LibWeb/Text/expected/wpt-import/css/css-nesting/nested-declarations-cssom.txt

@@ -2,17 +2,17 @@ Harness status: OK
 
 Found 12 tests
 
-6 Pass
+7 Pass
-6 Fail
+5 Fail
 Fail	Trailing declarations
 Fail	Mixed declarations
 Fail	CSSNestedDeclarations.style
 Pass	Nested group rule
 Pass	Nested @scope rule
 Fail	Inner rule starting with an ident
-Fail	Inserting a CSSNestedDeclaration rule into style rule
+Pass	Inserting a CSSNestedDeclaration rule into style rule
-Fail	Inserting a CSSNestedDeclaration rule into nested group rule
+Pass	Inserting a CSSNestedDeclaration rule into nested group rule
-Pass	Attempting to insert a CSSNestedDeclaration rule into top-level @media rule
+Fail	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
 Pass	Attempting to insert a CSSNestedDeclaration rule, all invalid declarations