Intent to prototype: ARIA Element Reflection

[Eitan Isaacson]

Summary: This allows ARIA relationship attributes to be set in JavaScript by directly referencing target elements. In particular, it will allow setting ARIA relationship attributes to work across Shadow DOM boundaries (with limitations).

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1769586

Specification:
 * https://html.spec.whatwg.org/multipage/common-dom-interfaces.html#reflecting-content-attributes-in-idl-attributes:element
 * The specific attributes to be implemented: https://w3c.github.io/aria/#ARIAMixin
 
 Platform coverage: desktop & mobile
 
 Preference: accessibility.ARIAElementReflection.enabled
 
 DevTools bug: The attributes should be exposed correctly in the DOM properties view, and changing them will be reflected in the accessibility panel.
 
 Link to standards-positions discussion: https://github.com/mozilla/standards-positions/issues/200
 
 Other browsers:
  * Safari: Shipped in Safari 16.4 via STP 151 https://developer.apple.com/documentation/safari-technology-preview-release-notes/stp-release-151
  * Chrome: Shipped behind `enable-experimental-web-platform-features`

 web-platform-tests: https://wpt.fyi/results/html/dom/aria-element-reflection.html

Cheers,

 Eitan.