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.