Expectations for aria-hidden and focused elements

[scottaohara]

This PR closes #1765 and is related to work that was done in #2037, but scoped only to the original issue I filed.

The intent of this PR is to identify not only how user agents would need to handle focusable elements that are aria-hidden (explicitly or due to being a descendant of an aria-hidden container) - but for the case where a focusable element is within an aria-hidden container, that the entire subtree would need to be re-exposed so that any other relevant information to the user could be made available. (e.g., so as to not just expose a "learn more" link, with no way to determine what someone would be learning about)

a simple example being like:

<div aria-hidden=true>
  <h3>Something or other</h3>
   some details about said something, or other.
  <a href=#>Learn more!</a>
</div>

• [ ] Related Core AAM Issue/PR:
• [ ] Related AccName Issue/PR:
• [ ] Any other dependent changes?

Test, Documentation and Implementation tracking

Once this PR and all related PRs have been approved by the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• [ ] Related APG Issue/PR:
• [ ] MDN Issue/PR:
• [ ] "author MUST" tests:
• [ ] "user agent MUST" tests:
• [ ] Browser implementations (link to issue or when done, link to commit):
  • WebKit:
  • Gecko:
  • Blink:
• [ ] Does this need AT implementations?

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