Proposal for popup attribute accessibility

Hi all, this is the current level of ideas/consensus that we have, and will
be discussing today: Popup A11y Proposal
<https://docs.google.com/document/d/1umackdZ9wZsr0cJtM6IfpFLN0RJKCypdYiRe_nJuk8c/edit#heading=h.7fodzd7pkxs0>
.

Note: I moved the content out of the wiki discussion because it was getting
hard to evolve a consensus document from all of the thread. We can move it
back to a wiki or create a different form if required/requested.

Feel free to add your thoughts via comments or reply to this email.

Aaron