Re: [w3ctag/design-reviews] HTMLPopupElement - <popup> (#599)

Hi @LeaVerou @hober @kenchris and @atanassov, wanted to share an update with you in case it's a good time to revisit this review. (Please let us know if a separate issue is preferable to reopening this one, thought it nice that the history was here)

`<popup>` itself is in active incubation in the Open UI CG: [initial proposal](https://open-ui.org/components/popup.research.explainer), [issues we are working through](https://github.com/openui/open-ui/issues?q=is%3Aissue+is%3Aopen+label%3Apopup), [first editor's draft PR](https://github.com/openui/open-ui/pull/355)

## CSS Anchored Positioning Scheme

* [An initial proposal](https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/CSSAnchoredPositioning/explainer.md) has been published. We have two viable proposals here for a path forward: the primary at-rules-based syntax, and an "Alternate A" syntax which uses a `position-set()` function within various property values. We think that actually Alternate A might be a bit better implementation-wise, as it avoids a style-on-layout dependency introduced by the at-rules syntax. However, we've invited author feedback on the ergonomics of the various options; TBD on an assessment there.
* The initial proposal aims to give enough flexibility to cover known use cases for anchored positioning. However, some authors may prefer "magic" over control (w/r/t repositioning anchored elements) and might find the current syntax somewhat overpowered for their use case. We plan to discuss how the proposals could evolve to empower 1) the developer who wants to express a preferred position and let the UA handle fallbacks and 2) the developer who has very specific requirements as to positioning-aware styling and fallback positioning logic.
* The proposal is limited to browser-managed, top-layer elements for now, due to perf reasons. We want to see what we can do to expand that scope, even if the use of this scheme is not completely unrestricted.
* Open UI has agreed to incubate this proposal/discussion and get it to a more mature place before formally bringing this proposal to CSSWG for specification. [CSSWG has been notified of this work](https://github.com/w3c/csswg-drafts/issues/5952#issuecomment-859079607). FYI, given that the reviewers are chairs/members of CSS: we plan to request a 5 minute agenda item presenting this scheme/inviting collaboration in Open UI. That will occur after yours truly is out of office next week. :)

Clearly, we're still early in co-designing a solution here for anchored positioning, but we wanted to prioritize giving the TAG a heads up. Thanks in advance!

cc / @mfreed7 @BoCupp-Microsoft

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