[csswg-drafts] [scroll-animations-1] Should switching from `animation-timeline: none` to `animation-timeine: --scroll-timeline` cause the animation to pause? (#13899)

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

== [scroll-animations-1] Should switching from `animation-timeline: none` to `animation-timeine: --scroll-timeline` cause the animation to pause? ==
(Related, but not identical to #13807 which concerns switching out of unresolved timelines)

Consider this [testcase](https://github.com/user-attachments/files/27495417/testcase.html).

There are two animations, one using the scroll timeline (`animation-timeline: --t1`), another using the document timeline (`animation-timeline: auto`). Clicking on one of the two buttons cause `animation-timeline` to be set to `none` for 500ms for the corresponding animation target.

* After clicking the "Toggle Document" button, the animation continues to play after the toggle.
* After clicking the "Toggle Scroll" button, the animation style applies, but is not responsive to scroll.

This seems inconsistent. I think this is because the play state of an animation with null timeline is paused, and while [setting the new timeline](https://drafts.csswg.org/web-animations-2/#setting-the-timeline), we fall into step 6 of "If to finite timeline."

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/13899 using your GitHub account


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Received on Thursday, 7 May 2026 20:29:50 UTC