[web-animations-2] Animate - Add more event handlers

Ref: https://drafts.csswg.org/web-animations-1/#the-animation-interface

Could these proposed event handlers be added to the Animation interface?

onstart

Example/proposal: https://jsfiddle.net/glenn2223/gj7heoLv/

Details
The event handler for each time the animation starts

Reason
In case someone wants to change the state it starts, as they can currently do when it finishes.
Currently, as per the example, setting any state before the calling play() causes issues when there is a delay or endDelay

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onAnimationStart - possible handler. Maybe a higher preference to the onKeyFrame handler?

Details
This would fire on (or just before) the first frame being initiated (or the implied first frame in the case of implicit to/from keyframes)

Reason
To fire an event the second that the animation actually starts (after delays)

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onAnimationFinish - possible handler. Maybe a higher preference to the onKeyFrame handler?

Details
This would fire on (or just after) the last frame being initiated (or the implied last frame in the case of implicit to/from keyframes)

Reason
To fire an event the second that the animation actually ends (before delays)

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onKeyFrame (index: number)

Proposal: https://jsfiddle.net/glenn2223/uwg589Lx/

Details
This would fire on each keyframe that is hit, passing the index of the keyframe

Reason
You can fire events at certain stages of the animation process as per the details in the proposal link just above.

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Also, is there a way to detect if it's actually running or just delayed? I saw the AnimationPlayState enumeration but that doesn't;t have a startDelay or endDelay state.

I only ask because if it's delayed I'd rather call cancel() but if it's running I'd rather call finish()

(Should I open a new issue)

Start with the Animation interface in the Web Animations specification and review the linked CSSWG draft and JSFiddle proposals. Compare the requested onstart, onAnimationStart, onAnimationFinish, and onKeyFrame handlers with the existing AnimationPlayState behavior. Done means the event-handler design and delayed-versus-running behavior are resolved in the specification.