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chromium/third_party/blink/web_tests/fast/selectors/querySelector-pseudo-slotted.html 

<!DOCTYPE html>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script src="../../shadow-dom/resources/shadow-dom.js"></script>
<div id="host1">
  <template data-mode="open">
    <slot name="slot1"></slot>
  </template>
  <span slot="slot1" id="span1"></span>
  <span slot="slot1" id="span2"></span>
</div>
<script>
convertTemplatesToShadowRootsWithin(host1);
document.body.offsetLeft;

test(() => {
  assert_equals(document.querySelector('::slotted(span)'), null);
  assert_equals(host1.shadowRoot.querySelector('::slotted(span)'), null);
}, "querySelector for ::slotted pseudo element");

test(() => {
  assert_equals(document.querySelectorAll('::slotted(span)').length, 0);
  assert_equals(host1.shadowRoot.querySelectorAll('::slotted(span)').length, 0);
}, "querySelectorAll for ::slotted pseudo element");
</script>