Tools/TestWebKitAPI/Tests/WebKit/beforeunload-slow.html - WebKit - Git at Google

 <!DOCTYPE html>
 <html>
 <head>
 <script>
 function sleep(millisecondsToWait) {
     var now = new Date().getTime();
     while (new Date().getTime() < now + millisecondsToWait) { }
 }

 onbeforeunload = (ev) => {
     sleep(70);
 }
 </script>
 </head>
 </html>
	<!DOCTYPE html>
	<html>
	<head>
	<script>
	function sleep(millisecondsToWait) {
	var now = new Date().getTime();
	while (new Date().getTime() < now + millisecondsToWait) { }
	}

	onbeforeunload = (ev) => {
	sleep(70);
	}
	</script>
	</head>
	</html>