The script tick ends up only processing JS callbacks related to animation
frames, so CSS transitions/animations end up not working as expected.
This could have accidentally worked before #12563 because we over-restyled, but
now this is no longer the case.
Other possible way to do it is making a layout reflow with RAF handle CSS
animations/transitions too, but that may not work if the reflow ends up being
suppressed (that could very well be the case), and we'd need to handle a lot
more state in the document, so this solution (assuming it doesn't break try)
seems a bit less flacky.
Fixes#12749.
805988e839