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chromium/third_party/blink/web_tests/external/wpt/html/browsers/browsing-the-web/unloading-documents/prompt/001.html 

<!doctype html>
<title>beforeunload event order</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<div id="log"></div>
<script>
var t = async_test();
events = [];
onload = t.step_func(function() {
  assert_array_equals(events, ["before src change", "beforeunload", "after src change"]);
  t.done();
})
</script>
<iframe src="001-1.html"></iframe>