ript type="module">
import { Polymer } from '../../polymer-legacy.js';
Polymer({
  is: 'x-foo'
});
</script>
  </dom-module>

  <dom-module id="x-nest">
    <template>
    <style>
      :host {
        --border: 4px solid tomato;
      }
    </style>
      <unknown-host id="unknown"></unknown-host>
    </template>
    <script type="module">
import { Polymer } from '../../polymer-legacy.js';
Polymer({
  is: 'x-nest',

  attached: function() {
    this.$.unknown.shadowRoot.appendChild(document.createElement('