Polymer 3

<!DOCTYPE html>
<html lang="en">
  <head>
    <script src="https://unpkg.com/@webcomponents/webcomponentsjs/webcomponents-loader.js"></script>
    <script type="module" src="custom-element.js"></script>
  </head>
  <body>
    <custom-element></custom-element>
  </body>
</html>

/**
 * @license
 * Copyright (c) 2018 The Polymer Project Authors. All rights reserved.
 * This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
 * The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
 * The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
 * Code distributed by Google as part of the polymer project is also
 * subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 */
import { PolymerElement, html } from 'https://unpkg.com/@polymer/polymer@3.0.0-pre.13/polymer-element.js';
// Define the class for a new element called custom-element
class CustomElement extends PolymerElement {
  
  static get template() {
    return html`
      <h1>Heading!</h1>
      <p>We are elements in <code>custom-element</code>'s local DOM.</p>
    `;
  }
}
// Register the new element with the browser
customElements.define('custom-element', CustomElement);