#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;
};