Summary: ElementInternals is a new addition to custom elements API, which allows developers to manage a custom element's internal states such as default ARIA role or ARIA label as well as having custom elements participate in form-associated behavior like submissions and validations.
Spec: https://html.spec.whatwg.org/multipage/custom-elements.html#the-elementinternals-interface
MDN: https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals
Other Resources:
=== ElementInternals and form-associated custom elements. ===