[csswg-drafts] [web-animations-1] Playing an animation does not account for an inactive timeline (#4379)

JTensai has just created a new issue for https://github.com/w3c/csswg-drafts:

== [web-animations-1] Playing an animation does not account for an inactive timeline ==
https://drafts.csswg.org/web-animations/#playing-an-animation-section

If an animation is supplied an inactive timeline (e.g. a ScrollTimeline using a scroller with style="display:none") it will fail.

Before setting `hold_time` in `play()` we need to first check that there is an active timeline. If not, we should not start the animation until the timeline has become active.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/4379 using your GitHub account