Issues with aria-expanded on tabpanel/tablist/tab #1032

I found this when researching for writing a new section in aria-practices for Controlling Widget States.

Here are the issues me and @spectranaut found:

• tabpanel - what is the advantage of using aria-expanded when aria-selected already indicates which tabs are selected?
• The spec is inconsistent about whether aria-expanded should go on tabpanel, tablist, or tab.

For a multi-selectable tablist, authors SHOULD ensure each visible tabpanel has its aria-expanded attribute set to true, and that the remaining hidden tabpanel elements have their aria-expanded attributes set to false.

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

For a multi-selectable tablist, authors SHOULD ensure each visible tabpanel has its aria-expanded attribute set to true, and that the remaining hidden tabpanel elements have their aria-expanded attributes set to false.

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

...but aria-expanded is a supported state on tab, not tabpanel or tablist

Originally posted by @zcorpan in #681 (comment)