Bug 275211 – Add `::details-content` pseudo-element

• Component: DOM
• Status: RESOLVED FIXED

[L. David Baron]

https://github.com/whatwg/html/pull/10265 is proposing changes to specify the rendering of the <details> element in more detail.  (See also https://github.com/WebKit/standards-positions/issues/351 .)

Some of these changes match existing implementations.  However, I'm aware of a number of things that would need to change in WebKit to match these changes:
* the existing changes in bug 157323 and bug 228843 would be needed to match what is already in the HTML spec
* the display property would need to be allowed to change more generally on both the <details> and also (I think) <summary> elements
* the ::details-content pseudo-element specified in https://drafts.csswg.org/css-pseudo-4/#details-content-pseudo would need to be implemented and made to match the correct element. (There may be some work here to implement the "part-like pseudo-element" concept.)
* the default display of the internal <slot> matched by the ::details-content pseudo-element would need to change to block rather than contents. (This is the one part here that involves compatibility risk, and it's possible we may need to revert this piece.)

[Radar WebKit Bug Importer]

<rdar://problem/129786929>

[Tim Nguyen (:ntim)]

Bug 157323 covers a lot of this (using display/content-visibility, stop forcing the display value, etc.), I'll use this bug to expose the ::details-content pseudo-element

[Tim Nguyen (:ntim)]

Pull request: https://github.com/WebKit/WebKit/pull/36786

[Tim Nguyen (:ntim)]

I filed bug 283446 to support chaining things after element backed pseudos. I have another PR that covers this (although ::first-line doesn't currently work) but wanted to get it landed separately.

[EWS]

Committed 286891@main (2e79c586fcac): <https://commits.webkit.org/286891@main>

Reviewed commits have been landed. Closing PR #36786 and removing active labels.