Detecting customelements define extends

extends.js

// Via https://github.com/ungap/custom-elements/blob/bdb7c27/index.js#L428-L456
const SUPPORTS_EXTENDING_BUILT_IN_ELEMENTS = (() => {
    const tagName = `extends-li-${Math.floor(Math.random() * 1000000)}`;
    self.customElements.define(tagName, class extends HTMLLIElement {}, { extends: 'li' });
    const elm = document.createElement('li', { is: tagName });
    return elm.outerHTML.indexOf(tagName) !== -1;
})();