src/third_party/web_platform_tests/dom/nodes/MutationObserver-disconnect.html

<!DOCTYPE HTML>
<meta charset=utf-8>
<title>MutationObservers: disconnect</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<h1>MutationObservers: disconnect</h1>
<div id="log"></div>
<section style="display: none">
<p id='n00'></p>
</section>
<script>
var n00 = document.getElementById('n00');
var parentTest = async_test("subtree mutations");
function masterMO(sequence, obs) {
  parentTest.step(function() {
    assert_equals(sequence.length, 4, "mutation records must match");
  });
  parentTest.done();
}
parentTest.step(function() {
  (new MutationObserver(masterMO)).observe(n00.parentNode, {"subtree": true, "attributes": true});
});

var disconnectTest = async_test("disconnect discarded some mutations");
function observerCallback(sequence, obs) {
  disconnectTest.step(function() {
    assert_equals(sequence.length, 1);
    assert_equals(sequence[0].type, "attributes");
    assert_equals(sequence[0].attributeName, "id");
    assert_equals(sequence[0].oldValue, "latest");
    disconnectTest.done();
  });
}

var observer;
disconnectTest.step(function() {
  observer = new MutationObserver(observerCallback);
  observer.observe(n00, {"attributes": true});
  n00.id = "foo";
  n00.id = "bar";
  observer.disconnect();
  observer.observe(n00, {"attributes": true, "attributeOldValue": true});
  n00.id = "latest";
  observer.disconnect();
  observer.observe(n00, {"attributes": true, "attributeOldValue": true});
  n00.id = "n0000";
});
</script>