LibWeb: Allow anonymous layer block rule with no declarations

Author: https://github.com/tcl3
Commit: 701fcb9e87
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5111
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -441,6 +441,7 @@ Optional<AtRule> Parser::consume_an_at_rule(TokenStream<T>& input, Nested nested
         .name = ((Token)input.consume_a_token()).at_keyword(),
         .prelude = {},
         .child_rules_and_lists_of_declarations = {},
+        .is_block_rule = false,
     };
 
     // Process input:
@@ -479,6 +480,7 @@ Optional<AtRule> Parser::consume_an_at_rule(TokenStream<T>& input, Nested nested
             // Consume a block from input, and assign the result to rule’s child rules.
             m_rule_context.append(rule_context_type_for_at_rule(rule.name));
             rule.child_rules_and_lists_of_declarations = consume_a_block(input);
+            rule.is_block_rule = true;
             m_rule_context.take_last();
 
             // If rule is valid in the current context, return it. Otherwise, return nothing.

Libraries/LibWeb/CSS/Parser/RuleParsing.cpp

@@ -288,7 +288,7 @@ Optional<FlyString> Parser::parse_layer_name(TokenStream<ComponentValue>& tokens
 GC::Ptr<CSSRule> Parser::convert_to_layer_rule(AtRule const& rule, Nested nested)
 {
     // https://drafts.csswg.org/css-cascade-5/#at-layer
-    if (!rule.child_rules_and_lists_of_declarations.is_empty()) {
+    if (rule.is_block_rule) {
         // CSSLayerBlockRule
         // @layer <layer-name>? {
         //   <rule-list>

Libraries/LibWeb/CSS/Parser/Types.h

@@ -31,6 +31,7 @@ struct AtRule {
     FlyString name;
     Vector<ComponentValue> prelude;
     Vector<RuleOrListOfDeclarations> child_rules_and_lists_of_declarations;
+    bool is_block_rule { false };
 
     void for_each(AtRuleVisitor&& visit_at_rule, QualifiedRuleVisitor&& visit_qualified_rule, DeclarationVisitor&& visit_declaration) const;
     void for_each_as_declaration_list(DeclarationVisitor&& visit) const;