Intent to Prototype: :heading & :heading(AnB#) selectors

[Keith Cirkel]

Summary:

  Adds the :heading and :heading(AnB#) selectors

Bug:

  https://bugzilla.mozilla.org/show_bug.cgi?id=1974386

Specification:

  https://drafts.csswg.org/selectors-5/#headings

Standards Body:

  W3C

Platform coverage:

  all.

Preference:

  layout.css.heading-selector.enabled

DevTools bug:

  N/A

Link to standards-positions discussion:

  N/A

Other browsers:

  Blink: No signal.

  WebKit: No Signal (https://github.com/WebKit/standards-positions/issues/523).

web-platform-tests:

  https://github.com/web-platform-tests/wpt/pull/53440.

I intend to implement `:heading`, which will be a pseudo class to match all heading elements (h1-h6), in addition it can take the functional form of `:heading(AnB#)` - where the comma separated list of AnB notations match a specific heading level (for example `:heading(1, 2n)`). 

This will make it easier for developers to style headings using a simpler selector, and will have stronger motivations if headings end up with dynamic levels such as the proposed `headingoffset` attribute (https://github.com/whatwg/html/issues/5033).