testing shadow root

components.test-shadow.js

import Ember from 'ember';

export default Ember.Component.extend({
  didInsertElement() {
    let shadow = this.element.attachShadow({mode: 'open'});

		this.set('shadow', shadow);
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
{{test-shadow}}

templates.components.test-shadow.hbs

{{#-in-element shadow}}
  <style>
    .internal {
      color: red;
    }
  </style>
  
  <div class='internal'>
  	hello!
    <a href='#'>Test</a>
  </div>
  
  {{yield}}
{{/-in-element}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}