This will work if aria-owns is used now that aria-owns mutates the tree. So we only need to cover the non-owns case here.

The spec notes that required owned elements (radio is a required owned element of radiogroup) can be descendants, not just children:

Any element that will be owned by the element with this role. For example, an element with the role list will own at least one element with the role listitem.

https://w3c.github.io/aria/#mustContain

An 'owned element' is any DOM descendant of the element, any element specified as a child via aria-owns, or any DOM descendant of the owned child.

https://w3c.github.io/aria/#terms

IMO, this vagueness around descendant is problematic. For example, what if the intervening div had a role of button? Maybe we could ignore intervening generic HyperTextAccessibles with non-presentation ARIA roles?

This also applies to other ARIA compound widgets. For example:

<div role="tree">
  <div tabindex="-1">
    <div role="treeitem">1</div>
  </div>
  <div tabindex="-1">
    <div role="treeitem">2</div>
  </div>
</div>

We special cased table rows in bug 1619383, but that was done in the table code, not in AccGroupInfo.

I think we can fix this by tweaking AccGroupInfo to walk siblings and parents such that it skips generic HyperTexts. We could write some helper functions and just call those instead of getting siblings/parents directly.

Transition the group position calculation - specifically the code in Update -
to Pivot such that we can handle finding siblings across intervening generic
container accessibles, such as SECTIONs. This fixes issues with finding the
values of "pos in set" and "set size" for descendant owned elements in compound
ARIA widgets. This revision also updates and adds tests for this functionality.

https://hg.mozilla.org/mozilla-central/rev/41bb66709957