[css-animations] Add a way to fill in a keyframe other than the 0% or 100% keyframe using the underlying style

[birtles]

Currently if you don't have a 0% or 100% keyframe, a keyframe is synthesized using the un-animated / underlying animated style (see #1841). It's odd, however, that this behavior only ever applies at the endpoints and you can never, for example, have a 50% keyframe where the property value is taken from the underlying style.

[AmeliaBR]

Could we overload the revert keyword just a little bit more, to give it special behavior in keyframes? It already has different behavior depending on the origin of the rule it's in, and animations are treated as a separate origin for the cascade.

[birtles]

I think revert might make sense for the un-animated style, but in #1841 I'm proposing that we use the underlying animation's style in which case I don't think revert makes sense (since we'd be applying other styles from the same origin).

[fantasai]

What about revert-layer? Essentially overloading each animation’s style rules as being a distinct “cascade layer”...

CC @mirisuzanne

[mirisuzanne]

revert-layer makes sense to me for this, at least on first glance.

[Loirooriol]

Already in the spec and ratified in #7083 (comment). Anything else to do?

[mirisuzanne]

I don't think there's anything more to do here, no.

[mirisuzanne]

Sorry, should have marked this first as pending commenter response. @birtles feel free to reopen if you think we need additional changes.