Polymer 3 TEMPLATE

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

import {Element as PolymerElement} from "https://unpkg.com/@polymer/polymer@3.0.0-pre.1/polymer-element.js"
  // Define the class for a new element called custom-element
  class CustomElement extends PolymerElement {
    static get properties () {
      return {
        prop1: {
          type: String,
          value: "Hi"
        }
      };
    }
    
    static get template() {
      return `
        <style>
        </style>
        [[prop1]]
        `
    }
  }
  // Register the new element with the browser
  customElements.define('custom-element', CustomElement);