chromium/third_party/blink/web_tests/xmlviewer/extensions-api.html

<html>
<head>
<script src="../resources/testharness.js"></script>
<script src="../resources/testharnessreport.js"></script>
<script>
async_test((t) => {
  let xmlWindow;

  t.add_cleanup(() => { xmlWindow.close(); });

  function checkSourceXML() {
    var sourceXML = xmlWindow.document.getElementById('webkit-xml-viewer-source-xml');
    assert_equals(typeof(sourceXML), 'object');
    assert_equals(sourceXML.firstChild.nodeName, 'root', 'Content of source XML.');

    t.done();
  }

  window.addEventListener("load", (e) => {
    xmlWindow = window.open('resources/sample.xml',
                            'XMLViewerTestWindow');
    xmlWindow.onload = t.step_func(checkSourceXML);
  });
}, 'XMLTreeConverted');
</script>
</head>
<body>
</body>
</html>