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chromium/third_party/blink/web_tests/shadow-dom/slots-fallback-2.html 

<!DOCTYPE html>
<script src='resources/shadow-dom.js'></script>
<div id='host'>
  <template data-mode='open'>
    <slot name='slot1'>
      <div id='fallback1'>fallback1</div>
      <slot name='slot2'>
        <div id='fallback2'>fallback2</div>
      </slot>
    </slot>
    <slot name='slot3'>
      <div id='fallback3'>fallback3</div>
      <slot name='slot4'>
        <div id='fallback4'>fallback4</div>
      </slot>
    </slot>
  </template>
  <div id='child1' slot='slot2'>child1</div>
</div>
<script>
convertTemplatesToShadowRootsWithin(host);
</script>