future proposal: use additive animations to decouple from other animations

[skeggse]

Per w3c/csswg-drafts#1594, the CSS WG is amenable to making animations composable/additive: either via a general-purpose !add annotation, or via an animation-composite property. The adoption and implementation of such functionality would remove the need for manually specifying the animation-name of an animation already attached to the desired element(s).

Our particular use-case for this feature is to interoperate with third-party pages from browser extensions without breaking animations the page may be using, and without hardcoding the animations the page already uses.