const CustomElement = class extends HTMLElement {}

customElements.define('custom-element', CustomElement);

​

const template = document.createElement('template');

template.innerHTML = '<custom-element></custom-element>';

​

const imported = document.importNode(template.content, true);

const cloned = template.content.cloneNode(true);

​

console.assert(imported.children[0].constructor === CustomElement, 'imported not upgraded');

console.assert(cloned.children[0].constructor === CustomElement, 'cloned was not upgraded');