LibWeb: Require CSS combinators to be followed by a simple selector

Author: https://github.com/Gingeh
Commit: a2cf1d17fd
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2478
Reviewed-by: https://github.com/AtkinsSJ

Libraries/LibWeb/CSS/Parser/SelectorParsing.cpp

@@ -166,8 +166,12 @@ Parser::ParseErrorOr<Optional<Selector::CompoundSelector>> Parser::parse_compoun
         simple_selectors.append(component.release_value());
     }
 
-    if (simple_selectors.is_empty())
+    if (simple_selectors.is_empty()) {
+        if (tokens.has_next_token() || combinator != Selector::Combinator::Descendant)
+            return ParseError::SyntaxError;
+
         return Optional<Selector::CompoundSelector> {};
+    }
 
     return Selector::CompoundSelector { combinator, move(simple_selectors) };
 }