LibWeb: Reject invalid background-position-* property values

Author: https://github.com/tcl3
Commit: dc58c11217
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3916
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CSS/Parser/PropertyParsing.cpp

@@ -1363,6 +1363,8 @@ RefPtr<CSSStyleValue> Parser::parse_single_background_position_x_or_y_value(Toke
                 transaction.commit();
                 return EdgeStyleValue::create(relative_edge, {});
             }
+            if (value->is_keyword())
+                return {};
         }
     }