Re: [csswg-drafts] [scroll-animations-1] blocking effects of timeline-scope (#8915)

Making `timeline-scope` block propagation of a named timeline allows subtrees to use named timelines without worrying about conflicts with ancestors using the same names. e.g.

```html
<div style="timeline-scope: --timeline">
  ....
  <div id="subtree" style="timeline-scope: --timeline">
    ...
    <!-- This is the --timeline used by anything inside of #subtree -->
    <div id="inner-timeline" style="view-timeline: --timeline;"></div>
  </div>
  ...
  <!-- This is the --timeline used by anything outside of #subtree -->
  <div id="outer-timeline" style="view-timeline: --timeline;"></div>
</div>
```

The downside is if you actually wanted to use the `#inner-timeline` named timeline you wouldn't have access to it without removing the intermediate `timeline-scope` declaration. Removing this would not break things in pages which did not separately define a timeline in the outer scope so as long as a developer controls the subtree it seems flexible.

IMO this allows for similar setups to custom variables where the `#subtree` can redefine a timeline such that it has a different timeline for that subtree.

If we don't block the propagation, the `--timeline` in the outer tree becomes invalid due to multiple timelines attaching to that name and so developers have to be careful to not use names that could conflict with an ancestor named timeline.

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Received on Thursday, 8 June 2023 00:10:12 UTC