This is happening because a button is not allowed to have children.

It's the same as writing this:

<button>
  <menu>
    <menuitem>
    <menuitem>
  </menu>
</button>

I don't think aria-owns is correct in this case. Perhaps aria-controls.

However, since I've seen this come up in examples maybe we need to handle it.

Interesting. Yeah, looks like an aria-controls type of case, but AFAIK aria-controls is barely implemented in any AT (JAWS, apparently?)

What would a good solution here look like, though? If the element with aria-owns tries to own something it can't, how do we expose that in a useful way? Does it just override the "no children" rule?

We could do something like: if you try to use aria-owns on an object that can't have children, they become siblings instead?

Easier would be to just ignore aria-owns in those cases, to avoid breaking sites like this

Regardless of whether it gets handled or not, it should remain in the tree. At the moment, it's simply gone. I would say keep it in its normal, unmodified position in the accessibility tree.

Agreed re: at least ignoring aria-hidden if it tries to put something somewhere it can't go.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/022687c2fb7ab9c92fdf42d4a185a8a0107a4f1e

commit 022687c2fb7ab9c92fdf42d4a185a8a0107a4f1e
Author: dmazzoni <dmazzoni@chromium.org>
Date: Tue Jun 28 20:15:36 2016

Aria-owns should not work on roles that can't have children.

BUG= 619641 

Review-Url: https://codereview.chromium.org/2072013004
Cr-Commit-Position: refs/heads/master@{#402515}

[add] https://crrev.com/022687c2fb7ab9c92fdf42d4a185a8a0107a4f1e/third_party/WebKit/LayoutTests/accessibility/aria-owns-ignores-leafs.html
[modify] https://crrev.com/022687c2fb7ab9c92fdf42d4a185a8a0107a4f1e/third_party/WebKit/Source/modules/accessibility/AXNodeObject.cpp