Add reference target to shadow root

[dandclark]

Reference Target is a feature to enable using 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. The main goal of this feature is to enable ARIA to work across shadow root boundaries.

In this change, add the referenceTarget property to ShadowRoot and add a definition of reference target that's exported for use in other specs.

See the reference target explainer.

• 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
  • 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
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

---

Preview | Diff