As animation support increases, it'd be great to have some sort of testing support for it.

We can't test them deteministically right now. Gecko has a "test mode" in their refresh driver that allows them to fake the times, advance the clocks, etc. as well as an API to get the computed style for an element in a given point of time.

We don't have that infra, and though it seems like a good amount of work, I think it's worth doing.