Chrome 151 / Firefox Nightly add headingoffset/headingreset HTML attrs behind flag

[Th3S4mur41]

Summary

Firefox and Chrome are prototyping headingoffset and headingreset attributes behind flags

The feature is currently available in:

• Firefox Nightly (153) with the flag dom.headingoffset.enabled
• Chrome 151 behind #enable-experimental-web-platform-features

Test results and supporting details

Verified support with the Following CodePen: https://codepen.io/editor/th3s4mur41/pen/019e8da3-90df-7090-ae14-3e4541413757

• Specs: https://html.spec.whatwg.org/multipage/sections.html#heading-levels-&-offsets
• Firefox Issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1974383
• Chromium Issue: https://issues.chromium.org/issues/333628468

[github-actions]

Tip: Review these changes grouped by change (recommended for most PRs), or grouped by feature (for large PRs).

[Th3S4mur41]

Thanks @caugner, I've applied the suggested changes

[caugner]

Let's add data for the corresponding HTMLElement properties at the same time:

• headingOffset
• headingReset

Implementations:

• Chrome implements them on Element: https://github.com/chromium/chromium/blob/48605b9955d48b2d1f1e1369ed3db666cc6f1214/third_party/blink/renderer/core/dom/element.idl#L194-L196
• Firefox implements them on HTMLElement as per the spec: https://searchfox.org/firefox-main/rev/4eb5d723d627edec42ca3e5d606e1227c656dfca/dom/webidl/HTMLElement.webidl#84-88

[Th3S4mur41]

Done, should we keep still keep them as global attributes, or only on corresponding Elements?

[caugner]

Done, should we keep still keep them as global attributes, or only on corresponding Elements?

Sorry, I should have been more precise:

• html.global_attributes.headingoffset refers to <h1 id="foo" headingoffset="…">,
• whereas api.HTMLElement.headingOffset refers to accessing the value in the DOM: document.getElementById('foo').headingOffset

[Th3S4mur41]

Thanks for the clarification, that makes sense.
Should there be a note regarding Chrome implementing it on Element considering this still might change?

[caugner]

Should there be a note regarding Chrome implementing it on Element considering this still might change?

Good point. There seems to be prior art, so it's worth adding a note ("Implemented on the `Element` interface instead of the `HTMLElement` interface."):

browser-compat-data/api/Element.json

Lines 6107 to 6110 in 1a19fb8

	"firefox": {
	"version_added": "1",
	"notes": "Before Firefox 35, it was implemented on the `Node` interface."
	},

It doesn't warrant setting partial implementation though.

[Th3S4mur41]

Done ☺️

[caugner]

Thank you! 🙌