Spec: https://html.spec.whatwg.org/C/#dom-attachinternals

Now ElementInternals doesn't work with customized built-in elements because of the first step of attachInternals().

1. If element's is value is not null, then throw a "NotSupportedError" DOMException.

We can't support Custom State Pseudo Class for customized built-in elements because of this restriction.

Proposal: Remove the step 1.

We need to triage all of IDL attributes/operations of ElementInternals.

• Attributes/operations for Form-associated custom elements
  Add "Throw "NotSupportedError" DOMException if target element's "is" value is not null." to them

• Default focus behavior ElementInternals: Add a capability to set the default focus behavior #5120
  Ditto?

• Default accessibility semantics for custom elements Allow setting default accessibility semantics for custom elements #4658
  I'm not sure if the current PR is reasonable for customized built-in elements. @domenic ?

• Custom State Pseudo Class
  It should work with customized build-in elements. We need to remove 'autonomous' from the specification.