WebRender currently (as of the last 2 weeks) renders this correctly, but slowly. Now that correctness is in, they can start looking at speed. Associated bug is https://bugzilla.mozilla.org/show_bug.cgi?id=1525420 , but it’s not a blocker for launching WebRender in the first place.

And no, this sort of thing isn’t particularly high priority. If lots of websites used these technologies to this extent then it’d be higher up the priority list I’m sure.