chromium/third_party/blink/web_tests/shadow-dom/slots-fallback-1.html

<!DOCTYPE html>
<script src='resources/shadow-dom.js'></script>
<div id='host'>
  <template data-mode='open'>
    <slot name='slot1'>
      <div>fallback1</div>
    </slot>
    <slot name='slot2'>
      <div>fallback2</div>
    </slot>
  </template>
  <div slot='slot1'>child1</div>
</div>
<script>
convertTemplatesToShadowRootsWithin(host);
</script>