[animation-triggers-1] Should play-forwards and play-backwards have no effect if the animation has already reached its end?

Currently, the table in animation-triggers-1 says the following above play-forwards and play-backwards:

The reason we wanted to have play-forwards and play-backwards was to support reversing effects, however, if according to the table we call play() when the effect is finished it sounds like we would restart the animation even if it has already reached its end. I think this is not at all what developers would expect.

E.g. suppose a developer sets up an animation on a button:

@keyframes press-animation {
  to: { transform: translate2d(-5px, 5px); }
}

button {
  animation: press-animation forwards;
  event-trigger: --press pointerdown, --depress pointerup;
  animation-trigger: --press play-forward --depress play-backward;
}

The intention is clearly that the button should animate to its pressed state, and back, however it may be possible to trigger the pointerup action multiple times (e.g. clicking both buttons one at a time and releasing one at a time). If the user does this, I think the developer would not intend the animation to replay the press animation from the start but rather to just stay pressed.

TLDR; I think we should not play the animation if it is finished after setting the playback rate for play-forwards and play-backwards

@ydaniv @DavMila

Note: I think it is not easy to create this degenerate situation with timeline triggers because we don't support multiple triggers yet and stateful triggers won't retrigger the exit state if they didn't first enter the entry state.

Start with the animation-triggers-1 shorthand table linked in the issue and review the 13-comment discussion about play-forwards and play-backwards in the finished state. Determine the intended behavior for already-finished animations, then update the specification wording and table once the semantics are resolved.