We probably want to merge https://chromium.googlesource.com/chromium/blink/+/96a74fcd42643e3e9b0320192b57a91e271f7709 Don't recalc style for the whole subtree during animations SyntheticStyleChange was used by animations to trigger a style recalc on the element that is being animated, but it was after FullStyleChange in the enum which meant we'd Force recalc style on all the descendants of the animating element as well even though it wasn't needed. Instead we can put SyntheticStyleChange earlier in the enum and only trigger a local style change on the animating element and let the normal Node::diff logic decide if we need to recalc our children. This should greatly improve performance when animating elements that contain a large number of descendants.
Need to make sure the regressions we saw from bug 79389 don't occur here too.