The web-animations-1 spec added a 'pending' member to the Animation interface:

https://drafts.csswg.org/web-animations-1/#dom-animation-pending

"pending, of type boolean, readonly
    Returns true if this animation has a pending play task or a pending pause task."

We need to add this to our Animation implementation. Unfortunately I don't think we have a very clear notion (or at least, I don't!) of when a pending play task or pending pause task is around.