implement headingoffset & headingreset attributes

[keithamus]

This specifies the headingoffset and headingstart attributes as proposed in #5033.

Closes #5033

• At least two implementers are interested (and none opposed):
  • Chromium: Actively prototyping.
  • Mozilla: (positive) Headingoffset & Headingreset attributes mozilla/standards-positions#1263 (comment)
  • WebKit: (no signal) Headingoffset & Headingreset attributes WebKit/standards-positions#523
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/333628468
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1974383
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=295092
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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/dom.html ( diff )
/index.html ( diff )
/infrastructure.html ( diff )
/sections.html ( diff )

[scottaohara]

thank you for working on this.

aside from the comments i left in my quick review, does it make sense to explicitly call out as a note that aria-level is not considered/taken into account with these attributes?

from re-reading the issue this stems from, i'm assuming people are likely thinking that if someone declares an aria-level on a heading, the value of that attribute is what the heading level will be exposed as, yes?

Or to be very clear, do people agree with the following:

<div headingoffset=1>
  <h1 aria-level=4> level 4 </h1>
  <h1> level 2 </h1>
  <h1 headingoffset=20 headingreset aria-level=3><!-- 3 --></h1>
</div>

if HTML doesn't have a note for this, then we can get it into html aam, aria in html, or both - and i'd assume MDN author guidance.

[domenic]

I haven't looked at the PR text itself yet, but I think such a note/example would be valuable.

[keithamus]

I've added a note mentioning aria-level as well as an example demonstrating their combinations.

[annevk]

Since this is not marked as draft and is still getting updates I'm going to make it extra clear that I consider #5033 (comment) to be blocking concerns.