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

<hr aria-label="Outside custom element">
<p>Hello, world</p>
<template id="test-contents">something</template>
<hr aria-label="Inside custom element">
<div role="group" aria-owns="t1 t2"></div>
<test-element role="button" id="t1"></test-element>
<test-element role="button" id="t2"></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>