#import 
// https://html.spec.whatwg.org/#customelementregistry
[Exposed=Window]
interface CustomElementRegistry {
    [CEReactions] undefined define(DOMString name, CustomElementConstructor constructor, optional ElementDefinitionOptions options = {});
    (CustomElementConstructor or undefined) get(DOMString name);
    Promise whenDefined(DOMString name);
    [CEReactions] undefined upgrade(Node root);
};
callback CustomElementConstructor = HTMLElement ();
dictionary ElementDefinitionOptions {
    DOMString extends;
};