Tools/TestWebKitAPI/Resources/cocoa/beforeunload.html - external/github.com/WebKit/webkit - Git at Google

 <!DOCTYPE html>
 <html>
 <head>
 <script>
 onbeforeunload = (ev) => {
     ev.preventDefault();
     return "foo";
 }
 </script>
 </head>
 </html>
	<!DOCTYPE html>
	<html>
	<head>
	<script>
	onbeforeunload = (ev) => {
	ev.preventDefault();
	return "foo";
	}
	</script>
	</head>
	</html>