[w3ctag/design-reviews] WebXR Dynamic Viewport Scaling TAG review request (#588)

HIQaH! QaH! TAG!

I'm requesting a TAG review of the dynamic viewport scaling feature that's part of WebXR.

Applications can optionally render to a subset of the WebXR viewport, using a scale factor that can be changed every animation frame. This is intended to be more efficient than resizing the full framebuffer which requires reallocation, and the UA can supply a recommended scale factor based on internal heuristics.

  - Explainer¹ (minimally containing user needs and example code): https://github.com/immersive-web/webxr/blob/master/explainer.md#dynamic-viewport-scaling

  - Specification URL: This is part of the WebXR API. Relevant sections include [requestViewportScale()](https://immersive-web.github.io/webxr/#dom-xrview-requestviewportscale), the [internal attributes](https://immersive-web.github.io/webxr/#view-viewport-modifiable) and the [obtain a scaled viewport](https://immersive-web.github.io/webxr/#xrview-obtain-a-scaled-viewport) algorithm.
  - Tests: https://github.com/web-platform-tests/wpt/blob/master/webxr/xr_viewport_scale.https.html and [results](https://wpt.fyi/results/webxr/xr_viewport_scale.https.html?label=experimental&label=master&aligned)
  - Security and Privacy self-review²: https://github.com/klausw/webxr/blob/master/designdocs/dynamic-viewport-scaling-privacy-security.md

  - GitHub repo (if you prefer feedback filed there): https://github.com/immersive-web/webxr/issues

  - Primary contacts (and their relationship to the specification):
      - Brandon Jones (@toji) Google, Editor
      - Kearwood Gilbert (@kearwood), (Former Mozilla), (Former?) Editor
      - Klaus Weidner (@klausw), Google
  - Organization(s)/project(s) driving the specification: Immersive-Web WG 
  - Key pieces of existing multi-stakeholder review or discussion of this specification: https://github.com/immersive-web/webxr/issues/1091

  - External status/issue trackers for this specification (publicly visible, e.g. Chrome Status): https://www.chromestatus.com/feature/5640976515203072


Further details:

  - [X] I have reviewed the TAG's [API Design Principles](https://w3ctag.github.io/design-principles/)
  - Relevant time constraints or deadlines: n/a
  - The group where the work on this specification is currently being done: [W3C Immersive Web](https://github.com/immersive-web)
  - The group where standardization of this work is intended to be done (if current group is a community group or other incubation venue): same
  - Major unresolved issues with or opposition to this specification: n/a
  - This work is being funded by: n/a

You should also know that...

This is an addition to the pre-existing WebXR specification editor's draft. TAG review was requested as part of [blink-dev feedback](https://groups.google.com/a/chromium.org/g/blink-dev/c/-khmtb4puL4/m/JWfSdqf9CAAJ).

There's strong developer interest in this feature including from <model-viewer> (see blink-dev thread above).

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

  ☂️ open a single issue in our GitHub repo **for the entire review**


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