LibWeb: Fail the parsing of an invalid HSL legacy syntax

Author: https://github.com/qligier
Commit: fac3ea77d8
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1915
Reviewed-by: https://github.com/AtkinsSJ ✅

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -2861,6 +2861,9 @@ RefPtr<CSSStyleValue> Parser::parse_hsl_color_value(TokenStream<ComponentValue>&
             inner_tokens.discard_whitespace();
 
             alpha = parse_number_percentage_value(inner_tokens);
+            // The parser has consumed a comma, so the alpha value is now required
+            if (!alpha)
+                return {};
             inner_tokens.discard_whitespace();
 
             if (inner_tokens.has_next_token())