[w3ctag/design-reviews] Scroll-linked Animations (#521)

Hello TAG!

I'm requesting a TAG review of Scroll-linked Animations. [Previous design review request](https://github.com/w3ctag/design-reviews/issues/330) for this stalled for lack of an explainer and the specification was less mature at that point. Since then we have moved the specification to be part of CSSWG and it has undergone major changes to ensure it integrates well with web-animations and improve the API ergonomics.

Scroll-linked animations are a common UX pattern on the web. We are proposing a new API that works in conjunction with existing [Web Animations](https://drafts.csswg.org/web-animations/) and [CSS Animations](https://drafts.csswg.org/css-animations/) APIs to enable declarative scroll-linked animations to improve their smoothness and ergonomics. In  Web Animations, an [AnimationTimeline](https://drafts.csswg.org/web-animations/#timelines) is the source of time progress that drives the animations. ScrollTimeline is a new timeline whose function is to translate “changes in scroll position” of an element into “changes in time”. This allows animations to be driven by scrolling as opposed to by wall clock thus taking advantage of all existing animation machinery on the web.

  - Explainer: https://github.com/w3c/csswg-drafts/blob/master/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](https://github.com/web-platform-tests/wpt/tree/master/scroll-animations)
  - Security and Privacy self-review²: [https://drafts.csswg.org/scroll-animations/#appendix-a-considerations-for-security-and-privacy](https://drafts.csswg.org/scroll-animations/#appendix-a-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, Mozilla
      - Antoine Quint, (garouts) Apple
      - Majid Valipour (majido), Google 
      - Olga Gerchikov (ogerchiko), Microsoft
  - 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)
      - [Chromium status](https://chromestatus.com/feature/6752840701706240)
      - [WebKit-dev thread](https://lists.webkit.org/pipermail/webkit-dev/2020-June/031228.html)

Further details:

  - [x] I have reviewed the TAG's [API Design Principles](https://w3ctag.github.io/design-principles/)
  - Relevant time constraints or deadlines: 
  - 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: We are working to remove `timeRange` in the timeline and there is [on-going discussions](https://github.com/w3c/csswg-drafts/issues/4862) on how-to achieve that.
  - This work is being funded by: See affiliation of editors.

You should also know that...

We have been working closely with web-animations specification (Brian Birtles is editor in both specification) to ensure the integration makes sense and works as expected. The specification is modest in terms of the usecases it tries to address but our hope is that it can be expanded in future to allow more usecases.

CSS syntax and Element-based offsets sections have been reworked most recently.

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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