#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; };