element-internals-focusability.md

I think we can get away with two states, with several caveats.

The states would be:

• focusable (or auto, or...)
• default (or unfocusable, or...)

The caveats:

• unfocusable custom elements would still be programmatically focusable.
• Safari would need to determine click/sequential focusability of focusable elements based on ElementInternals role.

https://gist.github.com/alice/bac0f6741b852f6e87f53e028efbde2f gives an example of using these two states to manage focus within a complex custom element.