With this, we pass the 8 ref tests in css/selectors/selectors-4/ which
previously failed. This is not technically a full implementation, as we
are supposed to first canonicalize the language range and tag, but that
will require downloading and processing the IANA language subtag
registry:
https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry
That's significantly more work, and WPT doesn't seem to test any cases
that require that, so we can leave it for now.
