Bugzilla

Form-associated custom elements don't have form IDL attribute. Instead, their ElementInternals object has a form IDL attribute. On getting, it must throw a "NotSupportedError" DOMException if the target element is not a form-associated custom element. Otherwise, it must return the element's form owner, or null if there isn't one.

Spec:
https://html.spec.whatwg.org/#association-of-controls-and-forms

Spec:
- https://html.spec.whatwg.org/#association-of-controls-and-forms
- https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#dom-elementinternals-form