…ttribute, animations, and XBL rules.

This also removes one of my FIXMEs from when I was looking at this code.

We don't seem to have a pre-existing test for this feature, sigh. I'll try to
write one if I have cycles for it...

Note that it not applying XBL rules is a feature, given the current state of
affairs. Video controls and such are right now unusable with no styles enabled.

Differential Revision: https://phabricator.services.mozilla.com/D4795