@polymer/polymer/test/unit/sub/x-test.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-test">
  <template>
    <x-sub id="sub">sub</x-sub>
  </template>
  <script>
    Polymer({
      is: 'x-test'
    })
  </script>
</dom-module>