[w3ctag/design-reviews] Review request on `blocking=render` attribute for scripts, stylesheets and link resources (Issue #727)

Braw mornin' TAG!

I'm requesting a TAG review of `blocking=render` attribute for scripts, stylesheets and link resources.

All current browsers already have a render-blocking mechanism: after a navigation, the user agent will not render any pixel to the screen before stylesheets and synchronous scripts are loaded and evaluated (or a UA-defined timeout is reached), to prevent a Flash of Unstyled Content and ensure that critical scripts are evaluated. We extend this idea by introducing the `blocking=render` attribute, which can explicitly mark other resources (`script`, `style` and `link` of types `modulepreload`, `preload` and `stylesheet`) as render-blocking, so that a flash of undesired contents can be prevented in more use cases (see explainer).

  - Explainer: https://gist.github.com/xiaochengh/9404abbecdc3b45c0e9f3d5e99fbc65d

  - Specification URL:
     - [Blocking attributes](https://html.spec.whatwg.org/multipage/urls-and-fetching.html#blocking-attributes)
     - [Render-blocking mechanism](https://html.spec.whatwg.org/multipage/dom.html#render-blocking-mechanism)
     - Please also check the references of the terms ["blocking rendering"](https://html.spec.whatwg.org/multipage/dom.html#block-rendering) and ["unblock rendering"](https://html.spec.whatwg.org/multipage/dom.html#unblock-rendering) for how different elements are affected by this new attribute
  - Tests: https://wpt.fyi/results/html/dom/render-blocking

  - User research: None
  - Security and Privacy self-review²:
    - https://gist.github.com/xiaochengh/9404abbecdc3b45c0e9f3d5e99fbc65d#file-security-self-review-md

  - Primary contacts (and their relationship to the specification):
      - Xiaochengh (@xiaochengh), Google, spec author and Chrome developer
  - Organization(s)/project(s) driving the specification: Google
  - Key pieces of existing multi-stakeholder review or discussion of this specification: 
      - https://github.com/whatwg/html/issues/7131

  - External status/issue trackers for this specification: https://chromestatus.com/feature/5452774595624960


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: April 22, 2022
  - The group where the work on this specification is currently being done: WHATWG
  - Major unresolved issues with or opposition to this specification: None
  - This work is being funded by: Google

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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