Polymer 2.0 - Place element html in the light dom instead of creating a shadow root

lightdom.html

<dom-module id="thing-thing">
  <template>
    <h1>I'm in the light dom</h1>
  </template>
  
  <script>
    class ThingThing extends Polymer.Element {
      _attachDom(dom) {
        this.appendChild(dom);
      }
    }
    
    customElements.define('thing-thing', ThingThing);
  </script>
</dom-module>

<!-- 
  instead of creating a shadow root in thing-thing, this just puts
  everything in the light dom so you should see something like...
-->
<thing-thing>
  <h1>I'm in the light dom</h1>
</thing-thing>

I would recommend against doing this and only use this if you are really struggling with getting styles from the document down into your components.