Add feature for focusable display: contents elements

[ddbeck]

Summary

Add a feature representing a specification change, where descendants of display: contents must be accessible.

Test results and supporting details

display: contents has severe accessibility issues (for example). It's difficult for web-features to generate a status for display: contents without data reflecting this fact. This PR attempts to add the data needed to accurately report support for display: contents in a non-surprising way.

Admittedly, this is a weird feature. No browser has actually shipped this "feature" yet, though there's work in progress in Chrome (Chrome Platform Status; intent to ship) and it's an Interop 2024 selection.

Alternatives considered:

• Representing this as a partial_implementation on css.properties.display.contents itself. Advantages include more aggressively warning developers about the seriousness of display: contents as it is currently implemented. The disadvantages include trying to communicate a rather large thing as an unstructured note and that it would (wrongly) imply that browsers have implemented display: contents incorrectly or non-interoperably, when it was the spec that changed.

• Representing this inversely as a deprecated feature (e.g., css.properties.display.contents.unfocusable_elements). This has a maintenance advantage: you get to remove this feature as irrelevant in a few years, after everyone has "removed" the old behavior. This has the disadvantage of implying that browsers have shipped some extra behavior, not that the behavior is, strictly speaking, wrong.

Links

• Allow elements with display:contents to be focusable. whatwg/html#9425
• css/css-display/focus/display-contents-focus.html on wpt.fyi
• display: contents accessibility web-platform-tests/interop#568
• Standards positions:
  • Focusability of elements with display:contents mozilla/standards-positions#772
  • focusability of elements with CSS display:contents WebKit/standards-positions#164

Related issues

• Safari Table Support - CSS Display Properties Break Tables. #19994
• Adds display: contents web-platform-dx/web-features#1966 (comment)

[caugner]

Overall, this change is plausible to me.

[caugner]

Requesting a second opinion from @Elchi3.

[Elchi3]

Thanks Daniel, for the careful consideration of sub feature vs. partial_implementation. I think a sub feature sounds good to me here, too.