ManualTests/quit-inside-unload.html - WebKit - Git at Google

 <script>
     var showedAlert = false;
     function handleUnload()
     {
         if (showedAlert)
             return;

         showedAlert = true;
         alert("Now Quit Safari. Do not dismiss this alert first.");
     }
 </script>
 <body onunload="handleUnload()">
     Close this window
 </body>
	<script>
	var showedAlert = false;
	function handleUnload()
	{
	if (showedAlert)
	return;

	showedAlert = true;
	alert("Now Quit Safari. Do not dismiss this alert first.");
	}
	</script>
	<body onunload="handleUnload()">
	Close this window
	</body>