The bison grammar for @supports should return 0 in case of an error.

Source/core/css/CSSGrammar.y.in

Index: Source/core/css/CSSGrammar.y.in
diff --git a/Source/core/css/CSSGrammar.y.in b/Source/core/css/CSSGrammar.y.in
index 7fc5abc5198e22b98407c2d95c022a511bd31634..ae8c0b1e4be3bdaa0aceabe904c0b4497f270f16 100644
--- a/Source/core/css/CSSGrammar.y.in
+++ b/Source/core/css/CSSGrammar.y.in
@@ -704,6 +704,7 @@ supports:
         $$ = parser->createSupportsRule($4, $9);
     }
     | before_supports_rule SUPPORTS_SYM supports_error {
+        $$ = 0;
         parser->endRuleBody(true);
         parser->popSupportsRuleData();
     }