Add support for running W3C CSS tests.

These are run through the same harness as the web-platform-tests.

tests/wpt/update_css.py

@@ -0,0 +1,40 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#!/usr/bin/env python
+
+import argparse
+import os
+import sys
+
+here = os.path.split(__file__)[0]
+
+def wpt_path(*args):
+    return os.path.join(here, *args)
+
+# Imports
+sys.path.append(wpt_path("harness"))
+from wptrunner import wptcommandline
+
+def update_tests(**kwargs):
+    from wptrunner import update
+
+    set_defaults(kwargs)
+    logger = update.setup_logging(kwargs, {"mach": sys.stdout})
+
+    rv = update.run_update(logger, **kwargs)
+    return 0 if rv is update.update.exit_clean else 1
+
+def set_defaults(kwargs):
+    if kwargs["config"] is None:
+        kwargs["config"] = wpt_path('config_css.ini')
+    wptcommandline.set_from_config(kwargs)
+
+def main():
+    parser = wptcommandline.create_parser_update()
+    kwargs = vars(parser.parse_args())
+    return update_tests(**kwargs)
+
+if __name__ == "__main__":
+    sys.exit(0 if main() else 1)