Add reference target

[dandclark]

Reference Target allows authors to specify an element inside a shadow root to be the target of any ID references referring to the host element. This would enable IDREF attributes such as for and aria-labelledby to refer to elements inside a component's shadow DOM while maintaining encapsulation of the internal details of the shadow DOM.

See the reference target explainer.

At a high level, the spec change consists of these parts:

• Define the concept of "resolving the reference target" on an element, following an IDREF from a host element to its target in the shadow, and potentially recursing.
• Update the get-the-attr-associated-element algorithm (and its array-of-elements form) to follow reference target.
• Centralize the definitions of ID reference attributes and list-of-ID-reference attributes as "element reference" and "set of element reference" attributes instead of repeating these definitions throughout the spec.
• Update uses of ID throughout the spec, where applicable, to use the new reference-target-aware get-the-attr-associated-element.
• Avoid exposing reference target elements in shadows from element-returning IDL properties by retargeting the result before returning.

See also the corresponding whatwg/dom#1353 which adds the definition of reference target used in this PR.
 

• At least two implementers are interested (and none opposed):
  • Reference Target for Cross-Root ARIA mozilla/standards-positions#1035 (no position stated yet)
  • Reference Target for Cross-Root ARIA WebKit/standards-positions#356 (no position stated yet)
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/shadow-dom/reference-target/tentative?label=experimental&label=master&aligned (still tentative)
  • PR to make non-tentative: Make reference target non-tentative web-platform-tests/wpt#51213
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/346835896/
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1952585
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=285634
• 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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