@polymer/polymer/test/unit/sub/x-sub.html

The Polymer library makes it easy to create your own web components. Give your element some markup and properties, and then use it on a site. Polymer provides features like dynamic templates and data binding to reduce the amount of boilerplate you need to

<dom-module id="x-sub">
  <template>
    <div id="test">test</div>
  </template>
  <script>
    class XSub extends Polymer.Element {
      static get is() { return 'x-sub' }
    }
    customElements.define(XSub.is, XSub);
  </script>
</dom-module>