Lets custom element authors use ElementInternals to set a custom element's default accessibility role, states, and properties, much like the default accessibility semantics for native elements.
There is currently no way for a custom element author to set default custom element accessibility semantics for a custom element. They can use AOM and modify ARIA attributes on the custom element directly, however these are user-visible and user-modifiable. An example of why this is bad: if the default attributes are modified by the user, and later on they remove the attribute, the default value would not be restored.