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chromium/third_party/blink/web_tests/virtual/permission-element-disabled/permission-disabled.html 

<!DOCTYPE html>
<meta charset=utf-8>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>

<!-- WHEN DISABLED, the permission element should behave as HTMLUnknownElement. -->

<div id=test><permission>TEXT</permission></div>

<script>
test(function(){
  const test = document.getElementById('test');
  assert_equals(test.childNodes.length,1,'Permission element should allow children');
  const permissionEl = test.firstElementChild;
  assert_true(permissionEl instanceof HTMLUnknownElement,'Permission element should be an HTMLUnknownElement');
  const permissionChild = permissionEl.firstChild;
  assert_true(permissionChild instanceof Text,'Permission element should allow children');
  assert_equals(permissionChild.data,'TEXT','Permission element should allow children');
}, "When the Permission element feature is disabled, <permission> should be an HTMLUnknownElement");
</script>