chromium/third_party/blink/web_tests/dom/domparsing/xmlserializer-xml-namespace.html

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <script src=../../resources/testharness.js></script>
    <script src=../../resources/testharnessreport.js></script>
</head>
<body>
    <div id="target"></div>
    <script type="text/javascript">
    test(() => {
        var target = document.getElementById("target");

        var xmlns = "http://www.w3.org/XML/1998/namespace";

        // XML with correct xml prefix on space
        var xml1 = document.createElementNS(xmlns, "foo");
        xml1.setAttributeNS(xmlns, 'xml:space', 'preserve');

        var xml2 = document.createElementNS(xmlns, "bar");
        xml2.setAttributeNS(xmlns, 'space', 'default');

        target.appendChild(xml1);
        target.appendChild(xml2);

        var serializer = new XMLSerializer();
        var xmlString = serializer.serializeToString(target);

        var outputText = document.createElement('div');
        outputText.textContent = xmlString;

        assert_equals(outputText.textContent, "<div xmlns=\"http://www.w3.org/1999/xhtml\" id=\"target\"><xml:foo xml:space=\"preserve\"/><xml:bar xml:space=\"default\"/></div>");
    }, 'XMLSerializer with xml namespace');
    </script>
</body>
</html>