[w3ctag/design-reviews] WG Revision: CSS Anchor Positioning Level 1 (Issue #1182)

fantasai created an issue (w3ctag/design-reviews#1182)

### Specification

https://www.w3.org/TR/css-anchor-position-1/

### Explainer

https://ishadeed.com/article/anchor-positioning/ 

### Links

- The WG's request for this TAG review: https://github.com/w3c/csswg-drafts/issues/13265
- TAG review of the previous version of this specification, if any: https://github.com/w3ctag/design-reviews/issues/848
- A description of what has changed since our previous review: Introduced `position-area` and `position-visibility` properties, introduced `anchor-center` alignment, renamed `anchor-default` to `position-anchor`, redesigned the fallback mechanism to use new `position-try-*` properties, and made many adjustments to layout details and algorithms. In short, the API has been thoroughly redesigned though the `anchor()` functions remain mostly intact.

### The specification

- [x] Follows the [Web Platform Design Principles](https://www.w3.org/TR/design-principles/).
- [x] Includes Security and Privacy Considerations sections based on answers to the [Security/Privacy Questionnaire](https://www.w3.org/TR/security-privacy-questionnaire/).

### Where and by whom is the work is being done?

- GitHub repo: https://github.com/w3c/csswg-drafts/issues
- Primary contacts:
  - Elika J. Etemad (@fantasai), Apple, co-editor, co-implementer
  - Tab Atkins, Jr. (@tabatkins), Google, co-editor
- Organization/project driving the specification: Google, Apple
- This work is being funded by: Google, Apple, Mozilla, Igalia
- Primary standards group developing this feature: CSSWG


### Feedback so far

- Active horizontal reviews: https://github.com/w3c/csswg-drafts/issues/13265
- Multi-stakeholder feedback: https://github.com/w3c/csswg-drafts/issues?q=label%3A%22css-anchor-position-1%22
- Major unresolved issues with or opposition to this specification: https://github.com/w3c/csswg-drafts/issues?q=label%3A%22css-anchor-position-1%22%20state%3Aopen
- Status/issue trackers for implementations: [Interop 2025 Subset](https://wpt.fyi/results/css/css-anchor-position?label=master&label=experimental&aligned&view=interop&q=label%3Ainterop-2025-anchor-positioning), [All Tests](https://wpt.fyi/results/css/css-anchor-position?label=experimental&label=master&aligned)

### You should also know that...

We're asking for a re-review of the spec because it has gone through substantial changes since the FPWD. At this point we have some detailed technical issues to work through, but expect the spec to go to CR largely in this form. The TAG may waive this review if you feel it is not needed.

Interesting points to consider from the TAG perspective are the points of integration with HTML, see in particular the `position-anchor` property as part of [determining the anchor box](https://www.w3.org/TR/css-anchor-position-1/#determining). We also ended up requiring the `position-area` property to resolve auto `margin` values to zero because this was the only way to address [confusing interactions](https://github.com/w3c/csswg-drafts/issues/12466) with the HTML default UA stylesheet for popovers in a backwards-compatible way. (It had been noted that these rules could cause integration problems when they were being developed, but anchor positioning wasn't developed enough to rely on it back then, so HTML pushed ahead with what they had.)

One of Mozilla's main concerns with this spec is the Layout & Style interleaving requirement for resolving `anchor()` values. See https://www.w3.org/TR/css-anchor-position-1/#anchor-pos This aspect was left out of Interop 2025 testing for that reason.

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