[w3ctag/design-reviews] [Review Request] New CSS @font-face descriptors for overriding font metrics (#557)

Saluton TAG!

I'm requesting a TAG review of new CSS descriptors `ascent-override`, `descent-override` and `line-gap-override` for @font-face rule. ([design doc](https://docs.google.com/document/d/1PW-5ML5hOZw7GczOargelPo6_8Zkuk2DXtgfOtJ59Eo/edit?usp=sharing))

Example usage:
```
@font-face {
  font-family: arial;
  src: local(Arial), url("http://example.com/arial.woff");
  ascent-override: 60%;
  descent-override: 40%;
  line-gap-override: 0%;
}
```

This allows web authors to override the metrics of a font and control text position within a line: the ascent, descent and line gap are set to exactly 60%, 40% and 0% of the em size. See also [demo](https://jsfiddle.net/wpnjav2k/) and its [video recording](https://www.youtube.com/watch?v=h_0T3SiISZg).

There are two major use cases:
1. Improve interoperability. Different browsers on different platforms currently use different pieces of data from a font file, causing [interop issues](https://github.com/w3c/csswg-drafts/issues/4792). This can be fixed by overriding the font metrics.
2. Reduce layout shifting caused by web fonts, by overriding the metrics of the fallback font to match the web font

  - Explainer¹ (minimally containing user needs and example code): as given above
  - Specification URL: Please refer to [design doc](https://docs.google.com/document/d/1PW-5ML5hOZw7GczOargelPo6_8Zkuk2DXtgfOtJ59Eo/edit#bookmark=id.7chgnuuzhh5g) for now. CSSWG has just [resolved](https://github.com/w3c/csswg-drafts/issues/4792#issuecomment-693528301) to add these descriptors and will add a spec soon
  - Tests: Currently as [Chromium web tests](https://chromium.googlesource.com/chromium/src/+/refs/heads/master/third_party/blink/web_tests/wpt_internal/css/css-fonts). Will upstream to WPT soon.
  - Security and Privacy self-review²: None
  - Primary contacts (and their relationship to the specification):
      - Xiaocheng Hu (@xiaochengh), Google
  - Organization(s)/project(s) driving the specification: CSSWG
  - External status/issue trackers for this specification: [Chrome status](https://chromestatus.com/feature/5651198621253632)

Further details:

  - [X] I have reviewed the TAG's [API Design Principles](https://w3ctag.github.io/design-principles/)
  - Relevant time constraints or deadlines: Oct 15, 2020 (Chrome M87 Beta Promotion)
  - The group where the work on this specification is currently being done: CSSWG
  - Major unresolved issues with or opposition to this specification: None

You should also know that...

[please tell us anything you think is relevant to this review]

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

  💬 leave review feedback as a **comment in this issue** and @-notify @xiaochengh



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