If you go to https://jsfiddle.net/birtles/zcjgvfy2/22/ and press "counter-clockwise", the animation runs clockwise. I didn't check to be sure, but I believe the call to reverse() has no effect on the accelerated animation.
This works properly in Firefox and Chrome. Still doesn't work in both latest Safari and TP.
I think there are two issues here:
1. we don't reflect the animation's playback rate for accelerated animations
2. we don't reflect changes to timing properties while an accelerated animation is already running
*** Bug 188841 has been marked as a duplicate of this bug. ***
For now the approach will be to bail if we're trying to animate something with playbackRate != 1. We'll try to use CA animations for that case in bug 211839.
Created attachment 399257 [details]
Committed r261637: <https://trac.webkit.org/changeset/261637>
All reviewed patches have been landed. Closing bug and clearing flags on attachment 399257 [details].