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):
  • Chromium is positive (I'd like to ship it there)
  • Reference Target for Cross-Root ARIA mozilla/standards-positions#1035 (Positive)
  • 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: Create documentation for Reference Target mdn/mdn#832
• The top of this comment includes a clear commit message to use.

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Preview | Diff

[dandclark]

@annevk , @smaug---- , this is ready for another review along with whatwg/html#10995. For context on the event propagation change, see WICG/webcomponents#1098. Thanks!

[smaug----]

What is the current testing situation? alice added several tests, but this is a rather complicated feature (at least from implementation point of view) so we need to ensure testing coverage is particularly good.
Gecko doesn't fail currently in too many (when run in Mozilla CI).
Chrome fails only some on wpt.fyi.

But are we testing all the complicated cases?

[dandclark]

I've tried to ensure that the WPTs for this are thorough; I and other contributors added quite a few during work on the Chromium prototype.

E.g. https://wpt.fyi/results/shadow-dom/reference-target/tentative/property-reflection.html and https://wpt.fyi/results/shadow-dom/reference-target/tentative/property-reflection-idl-setters.html test reflection behavior for all the properties affected by reference target, with references pointing to and from every HTML element type.

https://github.com/web-platform-tests/wpt/blob/master/shadow-dom/reference-target/tentative/event-path.html covers all the interesting event propagation scenarios discussed in WICG/webcomponents#1098, plus some others I thought of.

All of the platform features testable in WPTs that can be affected by reference target (form submission, commandfor, interestfor, <label> (implicit association and with for)), popovertarget, and aria-labelledby, have WPTs under https://wpt.fyi/results/shadow-dom/reference-target/tentative.

There are a few other ARIA properties affected by reference target that aren't yet testable in WPTs; there are tests for those in the Chromium source tree at https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/shadow-dom/reference-target/tentative/.

For some reason it doesn't seem like the Mozilla CI results are including the full set of tests. There are a larger number of tests in https://wpt.fyi/results/shadow-dom/reference-target/tentative that Gecko is shown as failing that don't appear in the linked Mozilla CI results.

[smaug----]

The pref in Gecko isn't enabled in Nightly, so wpt.fyi still shows many tests as failing. The pref is enabled in Mozilla CI.

[smaug----]

Looks reasonable, with the nits annevk has mentioned