Since the finished promise of an Animation is one and the same during the life of the animation, the Animation cannot be properly played multiple times, i.e. the following code fails:

animation1.play().finished
    .then(function () { return animation1.play().finished; })
    .then(function () { return animation1.play().finished; })

We should make the play() method return a new Promise each time it is played.

In other words, there should be a new promise returned for each play invocation -- not one promise per Animation instance.