LibWeb/CSS: Parse mask-mode property

Author: https://github.com/InvalidUsernameException
Commit: d022eea9fc
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5511
Reviewed-by: https://github.com/AtkinsSJ ✅
Reviewed-by: https://github.com/Calme1709

Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp

@@ -665,6 +665,7 @@ Parser::ParseErrorOr<NonnullRefPtr<CSSStyleValue const>> Parser::parse_css_value
             return parsed_value.release_nonnull();
         return ParseError::SyntaxError;
     case PropertyID::MaskComposite:
+    case PropertyID::MaskMode:
         if (auto parsed_value = parse_simple_comma_separated_value_list(property_id, tokens))
             return parsed_value.release_nonnull();
         return ParseError::SyntaxError;