LibWeb: Reject CSS values with unprocessed tokens / extra semicolons

Author: https://github.com/bplaat
Commit: 832bb978f5
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4543
Reviewed-by: https://github.com/AtkinsSJ ✅
Reviewed-by: https://github.com/gmta

Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp

@@ -370,7 +370,7 @@ Parser::ParseErrorOr<NonnullRefPtr<CSSStyleValue const>> Parser::parse_css_value
 
         if (token.is(Token::Type::Semicolon)) {
             unprocessed_tokens.reconsume_current_input_token();
-            break;
+            return ParseError::SyntaxError;
         }
 
         if (property_id != PropertyID::Custom) {