LibWeb: Treat a CSS declaration with a "--" property name as invalid

Author: https://github.com/tcl3
Commit: 7a6e4f5ec8
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5210
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -1125,6 +1125,8 @@ Optional<Declaration> Parser::consume_a_declaration(TokenStream<T>& input, Neste
 
     // 8. If decl’s name is a custom property name string, then set decl’s original text to the segment
     //    of the original source text string corresponding to the tokens of decl’s value.
+    if (is_invalid_custom_property_name_string(declaration.name))
+        return {};
     if (is_a_custom_property_name_string(declaration.name)) {
         // TODO: If we could reach inside the source string that the TokenStream uses, we could grab this as
         //       a single substring instead of having to reconstruct it.