chromium/content/test/data/accessibility/html/custom-element-with-aria-owns-inside-slot.html

<template id="test-contents">
  <div role="doc-endnotes">
    <div role="presentation">
      <span role="doc-endnote" id="e1">Endnote 1</span>
      <div role="doc-endnote" id="e2">Endnote 2</div>
      <slot name="stuff"></slot>
    </div>
  </div>
</template>
<test-element role="group" aria-owns="1 2">
  <!-- This content is rendered because it's assigned to a slot.
       This is important: aria-owns only works with rendered targets.
       See also custom-element-with-aria-owns-inside.html -->
  <div role="group" slot="stuff">
    <div role="presentation">
      <span role="button" id="1">1</span>
      <div role="button" id="2">2</div>
    </div>
   </div>
</test-element>
<script>
  class TestElement extends HTMLElement {
    constructor() {
      super();

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