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

Source/core/css/CSSGrammar.y.in

 /*
  *  Copyright (C) 2002-2003 Lars Knoll (knoll@kde.org)
  *  Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  *  Copyright (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *  Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  *  Copyright (C) 2012 Intel Corporation. All rights reserved.
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
@@ skipping 686 matching lines @@
   IDENT maybe_space {
       $$ = $1;
   }
   ;
 
 supports:
     before_supports_rule SUPPORTS_SYM maybe_space supports_condition at_supports_rule_header_end '{' at_rule_body_start maybe_space block_rule_list save_block {
         $$ = parser->createSupportsRule($4, $9);
     }
     | before_supports_rule SUPPORTS_SYM supports_error {
+        $$ = 0;
         parser->endRuleBody(true);
         parser->popSupportsRuleData();
     }
     ;
 
 supports_error:
     error ';' {
     }
     | error invalid_block {
     }
@@ skipping 1278 matching lines @@
     }
     ;
 
 error_location: {
         $$ = parser->currentLocation();
     }
     ;
 
 %%