[scroll-animations-2] consider [css-scoping] / `@scope` + `select()` for referencing scroller

[Schepp]

With more and more CSS features relying on somehow "tagging" elements for later reference, like in Scroll-linked Animations, and each time introducing new tagging vocabulary and mechanisms of their own, I wonder if it would make more sense to build upon @mirisuzanne's idea of css-scoping / issue #5809 / CSS Scoping and combine that with the previously used select() function?

So instead of writing this:

.scroll-container {
  scroll-timeline-name: foo;
}

.animated-sibling-or-child-of-scroll-container {
  animation-timeline: foo;
}

we could have this:

@scope(.ancestor-of-both) {
  .animated-sibling-or-child-of-scroll-container {
    animation-timeline: scroll(select(:scope .scroll-container));
  }
}

Drawback of this would be that another not even started idea/spec would block the finalization of this one (and potential others). A solution could be to release Scroll-linked Animations the way it is currently planned and to upgrade it later to also support a CSS Scoping aware select() function for referencing the scroll container.

[fantasai]

See #7759 I think that handles what you're asking for?

[flackr]

I think we can at least move this to level 2 for consideration whether there is a better way to generalize scoped selection?