Created attachment 327089 [details]
If animation-play-state is "paused" for a given animation on page load, and a style recomputation happens before it is changed to running, the animation properties no longer seem to apply to the element.
See this reproduction (I've also attached a simple html file): https://jsfiddle.net/6c63z10z/10/
I'm on macOS 10.12, but a colleague of mine said he sees the same issue in Safari 11 on high sierra.
In Chrome, Firefox, both boxes start spinning at once, 500ms after page load.
In Safari, one of two things will happen:
- At 500ms after page load, the first box will snap to horizontal, and at 1000ms, the right box will start to spin
- At 500ms after page load, both boxes will snap to horizontal, and neither will begin spinning
This is also present in Safari on iOSv11.1.1