[w3ctag/design-reviews] HTMLPopupElement - <popup> (#680)

Braw mornin' TAG! (That's a new one for me 😄 )

I'm requesting a TAG review of the `<popup>` element. This is effectively a re-opening of the [previously requested TAG review for this feature](https://github.com/w3ctag/design-reviews/issues/599).

That review was closed, waiting for the proposed "Anchor Positioning" feature to be available for review. Please see [this comment](https://github.com/w3ctag/design-reviews/issues/599#issuecomment-863632279) on the old review, linking to the explainer for Anchor Positioning, plus some additional context. Essentially, the Anchor Positioning feature now has an explainer and a significant amount of brainstorming is happening around the [shape of the API](https://docs.google.com/presentation/d/1g0kCtpbGHqzJybhrP1vgbQapXSZW3zMopYOjNfBf0OQ/edit#slide=id.p). From those links you should be able to get a sense for what will be possible, positioning-wise, for `<popup>`. And hopefully that's enough to re-open and continue the TAG review for `<popup>`. In the time since the last TAG review, a [draft specification has been written](https://open-ui.org/components/popup) and reviewed by editors of the WHATWG HTML spec. And a prototype implementation in Chromium is basically complete. Several of the comments on the prior TAG review lead to changes that have been made in the spec and implementation, e.g. the addition of an `initiallyopen` attribute to declaratively open the popup.

  - Explainer¹: https://open-ui.org/components/popup.research.explainer

  - Specification URL: https://open-ui.org/components/popup

  - Tests: https://wpt.fyi/results/html/semantics/interactive-elements/the-popup-element

  - User research: https://open-ui.org/components/popup.research

  - Security and Privacy self-review²: 
  - GitHub repo (if you prefer feedback filed there): 
  - Primary contacts (and their relationship to the specification):
      - Mason Freed (@mfreed7), Google (current feature champion)
      - Melanie Richards (@melanierichards), Netlify, previously Microsoft (previous champion, wrote spec draft)
  - Organization(s)/project(s) driving the specification: OpenUI, Google, Microsoft
  - Key pieces of existing multi-stakeholder review or discussion of this specification: [Prior TAG review](https://github.com/w3ctag/design-reviews/issues/599)
  - External status/issue trackers for this specification: https://github.com/openui/open-ui/issues?q=is%3Aissue+is%3Aopen+label%3Apopup


Further details:

  - [X] I have reviewed the TAG's [Web Platform Design Principles](https://w3ctag.github.io/design-principles/)
  - Relevant time constraints or deadlines: None
  - The group where the work on this specification is currently being done: [OpenUI](https://open-ui.org/)
  - The group where standardization of this work is intended to be done: WHATWG HTML/DOM
  - Major unresolved issues with or opposition to this specification: None that I know of
  - This work is being funded by: Google & Microsoft

You should also know that...

I appreciate the TAG. 👍 

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 @mfreed7


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