RFC: Reference Target for Cross-root ARIA

[behowell]

Reference Target is a new feature that enables creating ARIA links to elements inside a component's shadow DOM, while maintaining encapsulation of the internal details of the shadow DOM.

The full proposal is in this PR, and you can see a formatted version here:

📜 Reference Target for Cross-root ARIA

I'd really appreciate any feedback and comments! You can leave comments directly on the .md file in this PR.

Thank you to @alice, @annevk, and @chrisdholt for taking an earlier look at this proposal and giving feedback.

[behowell]

Apologies this has been sitting around for a while. I've made some updates to this proposal. Here's a summary:

1. Renamed the feature to "Reference Target" instead of "Reference Delegate". See https://github.com/WICG/aom/pull/207#discussion_r1445354205
2. Broke proposal into two phases. Phase 1 includes only the referenceDelegate attribute to try to get that feature in faster. Phase 2 can come later to give more bake time for the referenceDelegateMap attribute, or some alternative.
3. Made updates based on PR feedback, and for clarity.

It would be great to get this explainer merged into the AOM repo once people have had a chance to look at the edits. Thank you for the reviews!

cc @alice, @nolanlawson, @Westbrook, @annevk, @asyncLiz

[behowell]

Thank you for the feedback everyone! I'm going to merge this PR to get the explainer checked in. Further feedback can be in the form of an issue in the aom repo. Thanks!

[alice]

Nice one! Thanks for all your work on this!