Add support for running W3C CSS tests.

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

tests/wpt/run_css.py

@@ -0,0 +1,19 @@
+# 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 run
+
+paths = {"include_manifest": run.wpt_path("include_css.ini"),
+         "config": run.wpt_path("config_css.ini")}
+
+def run_tests(**kwargs):
+    return run.run_tests(paths=paths, **kwargs)
+
+def main():
+    return run.main(paths)
+
+if __name__ == "__main__":
+    sys.exit(0 if main() else 1)