src/third_party/web_platform_tests/XMLHttpRequest/event-error.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>XMLHttpRequest Test: event - error</title>
<link rel="author" title="Intel" href="http://www.intel.com">
<meta name="assert" content="Check if event onerror is fired When the request has failed.">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<div id="log"></div>

<script>

async_test(function (t) {
  var client = new XMLHttpRequest();
  client.onerror = t.step_func(function(e) {
    assert_true(e instanceof ProgressEvent);
    assert_equals(e.type, "error");
    t.done();
  });

  client.open("GET", "http://example.nonexist");
  client.send("null");
}, document.title);

</script>