Expectations for aria-hidden and focused elements

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>
```

Author: scottaohara

index.html

@@ -11604,6 +11604,7 @@ <h2>Definitions of States and Properties (all aria-* attributes)</h2>
 				<p><a>Indicates</a>, when set to <code>true</code>, that an <a>element</a> and its entire subtree are hidden from assistive technology, regardless of whether it is visibly rendered.</p>
 				<p>User agents determine an element's [=element/hidden=] status based on whether it is rendered, and the rendering is usually controlled by CSS. For example, an element whose <code>display</code> property is set to <code>none</code> is not rendered. An element is considered [=element/hidden=] if it, or any of its ancestors are not rendered or have their <code>aria-hidden</code> attribute value set to <code>true</code>.</p>
 				<p>Authors MAY, with caution, use aria-hidden to hide visibly rendered content from assistive technologies <em>only</em> if the act of hiding this content is intended to improve the experience for users of assistive technologies by removing redundant or extraneous content. Authors using aria-hidden to hide visible content MUST ensure that identical or equivalent meaning and functionality is exposed to assistive technologies.</p>
+				<p>Additionally, authors SHOULD ensure that any elements that are accessibility descendants of an aria-hidden element, or any elements which have been marked as aria-hidden themselves, are prevented from receiving focus. If an aria-hidden element or a descendant of an aria-hidden ancestor receives focus, User Agents MUST ignore the aria-hidden state of any ancestor to the focused element, resulting in the entire subtree to be exposed to assistive technologies from that point forward, even once the element loses focus or is no longer focusable.</p>
 				<p class="note">Authors are advised to use extreme caution and consider a wide range of disabilities when hiding visibly rendered content from assistive technologies. For example, a sighted, dexterity-impaired individual might use voice-controlled assistive technologies to access a visual interface. If an author hides visible link text "Go to checkout" and exposes similar, yet non-identical link text "Check out now" to the accessibility API, the user might be unable to access the interface they perceive using voice control. Similar problems can also arise for screen reader users. For example, a sighted telephone support technician might attempt to have the blind screen reader user click the "Go to checkout" link, which they might be unable to find using a type-ahead item search ("Go to…").</p>
 				<p class="note">As of ARIA 1.3, <code><sref>aria-hidden</sref>="false"</code> is now synonymous with <code>aria-hidden="undefined"</code>.</p>
 				<p class="note">The original intent for <code>aria-hidden="false"</code> was to allow user agents to expose content that was otherwise hidden from the accessibility tree. However, due to ambiguity in the specification and inconsistent browser support for the <code>false</code> value, the original intent is no longer supported.</p>