chromium/content/test/data/accessibility/html/custom-element.html

<hr aria-label="Outside custom element">
<div id="test-contents">
  <div role="presentation">
    <span>a</span>
    <span style="display:none">b</span>
    <span style="visibility:hidden">c</span>
  </div>
</div>
<hr aria-label="Inside custom element">
<test-element></test-element>
<script>
  class TestElement extends HTMLElement {
    constructor() {
      super();

      const testContents = document.getElementById('test-contents');
      this.attachShadow({mode: 'open'}).innerHTML = testContents.outerHTML;
    }
  }
  customElements.define('test-element', TestElement);
</script>