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chromium/third_party/blink/web_tests/external/wpt/dom/nodes/Element-childElement-null.html 

<!DOCTYPE HTML>
<meta charset=utf-8>
<title>Null test</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<h1>Test of firstElementChild and lastChildElement returning null</h1>
<div id="log"></div>
<p id="parentEl" style="font-weight:bold;">Test.</p>
<script>
test(function() {
  var parentEl = document.getElementById("parentEl")
  assert_equals(parentEl.firstElementChild, null)
  assert_equals(parentEl.lastElementChild, null)
})
</script>