chromium/src/third_party/blink/web_tests/dom/document_fragment/sibling-rules-under-document-fragment.html

<title>Tests :first-child rule under DocumentFragment</title>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script>
setup({single_test: true});

var fragment = document.createDocumentFragment();
var div = document.createElement('div');
var a1 = document.createElement('a');
var a2 = document.createElement('a');

fragment.appendChild(a1);
div.appendChild(a2);

var fragmentSelected = fragment.querySelector('a:first-child');
var elementSelected  = div.querySelector('a:first-child');

assert_equals(fragmentSelected, a1);
assert_equals(elementSelected, a2);
done();
</script>