`headingoffset`, `headingreset` attributes & `:heading`, `:heading(N)` pseudo classes

Summary: A system for creating structured headings within a document.

Intent https://groups.google.com/a/chromium.org/g/blink-dev/c/8yl-pJhuLHE/m/0l7fGKgaAAAJ

[wip] Spec: https://github.com/whatwg/html/pull/11086

5445406: Prototype 'headingoffset'/`headingreset` attributes on HTMLElement.: https://chromium-review.googlesource.com/c/chromium/src/+/5445406

Other Resources:

=== This is a demo of `headingoffset`, `headingreset` attributes & `:heading`, `:heading(N)` pseudo classes ===

=== This is a demo of `headingoffset`, `headingreset` attributes are only prototyped in Chrome Dev 136. pseudo classes are still under construction ===

Settings

Account

Not a Modal Dialog

This is not a Modal Dialog, headingreset is not applied and taken control by the heading level algorithm.

So, the heading level remains as h2.

see WPT: https://chromium-review.googlesource.com/c/chromium/src/+/5445406/23/content/test/data/accessibility/html/heading-with-headingoffset-dialog.html

Modal Dialog

This is a Modal Dialog, headingreset is applied.

So, the heading level is reset and now h1.
headingoffset is properly applied, and h1 is converted to h2