Allow headings in <summary> elements for <details>

[yatil]

Currently, headings in <summary> elements are not allowed and do not show up in the accessibility tree as headings. For accordions and other use cases, using headings inside the <summary> makes a lot of sense. Navigating to a heading inside the summary should set the focus to the summary and then have the expected interaction to open and close the details element.

[zcorpan]

@whatwg/a11y are there any issues with AT with allowing headings in summary?

[zcorpan]

I think the issue is that summary is exposed as a button in the a11y tree, and buttons can only have plain text. So long as that is the case we can't allow headings (same as with button element).

[scottaohara]

headings are presently allowed, but per the way this has been implemented they don't always get exposed. this is related to #2272

[janbrasna]

FYI it's more of an AT issue with more appropriate role for this use case missing, see the discussions around this topic in general: w3c/aria#939 (comment)

[prlbr]

Headings in <summary> have been allowed for about a decade at least. That is they have been valid for as log as the Internet Archive's Wayback Machine reaches back for the currently relevant spec page: https://web.archive.org/web/20141013114912/https://html.spec.whatwg.org/multipage/forms.html#the-summary-element