Per: HTML issue 4658 and ARIA issue #984 it should be noted that a custom element may have no corresponding role (map to generic) or may have a role set by ElementInternals.

If the role is set in that way, and the custom element doesn't merely act as a container for child elements (light or shadow dom) with their own roles, then the listing for the custom element would need to indicate that it either maps to generic, OR the role set by ElementInternals.

In that case, the allowed state/properties/naming/etc. would be dependent on the role the element represents.