The web-animations-1 spec added an 'updatePlaybackRate' method to the Animation interface:

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

"void updatePlaybackRate(playbackRate)

Performs an asynchronous update of the playback rate of this animation by performing the seamlessly update the playback rate procedure, passing playbackRate as the new playback rate."

We need to add this to our Animation implementation. This also requires checking whether setting our playbackRate member directly causes a synchronous or asynchronous update, as it is now meant to be synchronous:

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

"Setting this attribute performs a synchronous update to the playback rate meaning that it does not make any attempt to synchronize with the playback state of animations running on a separate process or thread. As a result, setting the playbackRate for an in-flight animation may cause it to jump."