I was going to send a test for `:focus` via wpt, but then realized it was
probably not spec-compliant with the new rules people want to follow for
:focus, so I filed w3c/csswg-drafts#4555 instead.

Testing `:hover` / `:active` via wpt looked quite a bit of a hassle.

Differential Revision: https://phabricator.services.mozilla.com/D55591