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chromium/third_party/blink/web_tests/fast/dom/setAttributeNS.html 

<!DOCTYPE html>
<script src="../../resources/js-test.js"></script>

<script>
description("Test that setAttributeNS with a null namespaceURI actually sets it to the value null and not a string with the contents 'null'.");

window.elem = document.createElementNS('http://www.example.org', 'test');
elem.setAttributeNS(null, 'name', 'value');
shouldBeNull('elem.attributes[0].namespaceURI');
</script>