shadow-dom/declarative/support/declarative-child-frame.html

<div id=iframe>
  <div id=wrapper>
    <div id=host>
      <template shadowroot=open>
        <span id=content>Content</span>
      </template>
    </div>
  </div>
</div>

<script>
function sendStatus(error, hasShadow, msg) {
  const name = window.name;
  parent.postMessage({ name, error, hasShadow, msg }, '*');
}

window.addEventListener('load', () => {
  const host = document.querySelector('#host');
  if (!host)
  	return sendStatus(true, false, 'Unable to find host element');
  return sendStatus(false, !!host.shadowRoot);
});
</script>