AnimationTrigger Play Methods will not auto-rewind, i.e. will not restart a finished animation. "play", "play-forwards" and "play-backwards" are values that, when specified in the association between an AnimationTrigger and an Animation, instructs the trigger to play the animation. This Chromestatus feature covers a specific aspect of the behavior of these keywords: when the relevant animation has already run to completion and these actions (play, play-forwards, play-backwards) are invoked, they will not cause the animation to restart, i.e. the animation will not "auto-rewind."
"play-forwards" and "play-backwards" cause an animation to play with positive and negative playback rate respectively. They are intended to support use cases where authors want "opposite" events on a page to be accompanied by similarly opposite visual effects. For example, an author might want to associate pointerdown and pointerup with swelling and shrinking respectively. In the event of multiple pointerdown events being observed before a pointerup, the more common scenario is that the first pointerdown triggers the swelling animation and subsequent pointerdowns do nothing until the next pointerup, which itself triggers the reversed (shrinking) animation. This is achieved by specifying that these keywords do not "auto-rewind", i.e. they do not restart a finished animation. For consistency, this auto-rewind behavior applies to all the play* keywords.