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chromium/third_party/blink/web_tests/external/wpt/html/browsers/browsing-the-web/unloading-documents/prompt/manual-002.html 

<!doctype html>
<title>Prompt when beforeunload has returnValue set</title>
<script>
addEventListener("beforeunload",
function(e) {e.returnValue = "PASS if you see this"},
false);
</script>
<p>When clicking the link below, you should get a prompt asking if you want to unload the document</p>
<a href="next.html">Click here</a>