chromium/third_party/blink/web_tests/external/wpt/shadow-dom/slot-fallback-content-001.html

<!DOCTYPE html>
<meta charset="utf-8" >
<meta name="author" title="Di Zhang" href="mailto:[email protected]">
<meta name="assert" content="Child nodes in host are default assigned to unnamed slot, when dynamically created.">
<title>Shadow DOM: Slots and fallback contents</title>
<link rel="match" href="slot-fallback-content-001-ref.html">

<p>Test passes if there are two lines of text "A", "B" below.</p>

<div id="host">
  <slot id="slot1">FAIL</slot>
  <div id="A">A</div>
  <div id="A">B</div>
</div>

<script>
// Both divs A and B overwrite fallback contents of default slot
const shadowRoot = host.attachShadow({ mode: "open" });
shadowRoot.appendChild(slot1);

</script>