This is still fairly incomplete (i.e. no capturing, etc), but it allows
a transition to "start", and then finish (on the next frame always, for
now) or timeout, appropriately.
I think it's in a reviewable shape, given that. There's one known
divergence from the spec, which is described in
https://github.com/w3c/csswg-drafts/issues/10822
Differential Revision: https://phabricator.services.mozilla.com/D220843
83a5b7cb8d