LibWeb/CSS: Treat repeated text-decoration-line values as invalid

Author: https://github.com/tcl3
Commit: 26293114d8
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5089
Reviewed-by: https://github.com/gmta ✅

Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp

@@ -3537,7 +3537,7 @@ RefPtr<CSSStyleValue const> Parser::parse_text_decoration_line_value(TokenStream
                 includes_spelling_or_grammar_error_value = true;
             }
             if (style_values.contains_slow(value))
-                break;
+                return nullptr;
             style_values.append(move(value));
             continue;
         }