Created attachment 446039 [details] reproduction Steps to reproduce: I have attached a screencast of this: 1. Go to https://codepen.io/emattias/pen/NWaxYrp?editors=0110 2. Press "toggle start animation button" 3. See the leftmost column animate in without a problem 4. Press "css transition on" button 5. Press "toggle start animation button" again 6. See that it will apply the transition set in the css when the javascript animation is cancelled (with .cancel() method) I can reproduce this in the latest firefox and safari versions but not in chrome. I think the way chrome handles it (not applying css transition) is what I prefer and expect to happen (since the width has not changed).
<rdar://problem/86412069>
Have you also filed a bug on Chromium? I haven't looked into whether Chromium was right and Firefox+Safari was wrong, but given they are the only engine with a different implementation, it would be good to get their take on this as well.
I created a chromium bug for this here: https://bugs.chromium.org/p/chromium/issues/detail?id=1279736