[w3ctag/design-reviews] Spec review for Scroll-driven Animations (Issue #828)

I'm requesting a TAG review of Scroll driven animation. This was [previously reviewed](https://github.com/w3ctag/design-reviews/issues/521) as the scroll linked animations spec but has since undergone a quite substantial rewrite in response to https://github.com/w3c/csswg-drafts/issues/6674 with the goal of being easier to author reusable CSS scroll driven animations by using scoped named animations in a similar fashion to container names. The other major change is that element based animations are now driven using the ViewTimeline concept. 

This specification defines mechanisms for driving the progress of an animation based on the scroll progress of a scroll container. These scroll-driven animations use a timeline based on scroll position, rather than one based on clock time. This module provides both an imperative API building on the Web Animations API and a declarative API building on CSS Animations.

  - Explainer¹: https://github.com/w3c/csswg-drafts/blob/main/scroll-animations-1/EXPLAINER.md
  - Specification URL: https://drafts.csswg.org/scroll-animations/
  - Tests: https://github.com/web-platform-tests/wpt/tree/master/scroll-animations
  - User research: [url to public summary/results of research]  N/A
  - Security and Privacy self-review²: https://github.com/w3c/csswg-drafts/blob/main/scroll-animations-1/EXPLAINER.md#considerations-for-security-and-privacy
  - GitHub repo (if you prefer feedback filed there): https://github.com/w3c/csswg-drafts/
  - Primary contacts (and their relationship to the specification):
      - Brian Birtles (@birtles), Invited Expert
      - Botond Ballo (@theres-waldo), Mozilla
      - Antoine Quint, (@graouts) Apple
      - Olga Gerchikov (@ogerchikov), Microsoft
      - Elika J. Etemad  (@fantasai), Invited Expert)
      - Robert Flack (@flackr), Google
  - Organization(s)/project(s) driving the specification: CSSWG
  - Key pieces of existing multi-stakeholder review or discussion of this specification:
  - External status/issue trackers for this specification (publicly visible, e.g. Chrome Status):
      - [Mozilla standard positions](https://github.com/mozilla/standards-positions/issues/347) ([implementation bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1676780))
      - [Chrome status](https://chromestatus.com/feature/6752840701706240)
      - [webKit-dev position](https://lists.webkit.org/pipermail/webkit-dev/2020-June/031228.html), new [webkit position request](https://github.com/WebKit/standards-positions/issues/152) filed

Further details:

  - [x] I have reviewed the TAG's [Web Platform Design Principles](https://www.w3.org/TR/design-principles/)
  - Relevant time constraints or deadlines: N/A
  - The group where the work on this specification is currently being done: CSSWG
  - The group where standardization of this work is intended to be done (if current group is a community group or other incubation venue): N/A
  - Major unresolved issues with or opposition to this specification: None known of
  - This work is being funded by: See affiliation of editors.

You should also know that...

In the [previous TAG review](https://github.com/w3ctag/design-reviews/issues/521) a few issues were raised:

There were [concerns](https://github.com/w3ctag/design-reviews/issues/521#issuecomment-657984261) about some of the use cases (which are admittedly already done today via script) being known triggers for vestibular disorders. We split this out into [#5321](https://github.com/w3c/csswg-drafts/issues/5321) where we explored ideas and drafted a proposal [#7440](https://github.com/w3c/csswg-drafts/issues/7440) for how we could work on addressing this concern more broadly for all animations. It's important to note that without declarative scroll driven animations, short of disabling Javascript the browser can't prevent these cases today - so the existence of this API puts us in a better position to address motion concerns in an automated way across more use cases.

There were also [concerns with explicit element id references](https://github.com/w3ctag/design-reviews/issues/521#issuecomment-921027219). In [#6674](https://github.com/w3c/csswg-drafts/issues/6674) the spec underwent a significant rewrite such that animation references could be established via CSS using the DOM hierarchy in a way that is easily reusable across lists of elements.

At this point in time, we have worked through resolutions to the majority of the issues in the [project tracker](https://github.com/w3c/csswg-drafts/projects/33). While the spec edits have not completely caught up it is in line with the current thinking and broadly explains the feature.

We'd prefer the TAG provide feedback as (please delete all but the desired option):

  🐛 open issues in our GitHub repo for **each point of feedback**

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