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

<!DOCTYPE html>
<script src='resources/shadow-dom.js'></script>
<div id='d1'>
  <template data-mode='open'>
    <div>d1-1</div>
    <slot name='d1-s1'></slot>
    <div>d1-2</div>
  </template>
  <div slot='d1-s1'>d2</div>
</div>
<script>
convertTemplatesToShadowRootsWithin(d1);
</script>