<!doctype html>
<title>Prompt on form submit</title>
<script>
addEventListener("beforeunload",
function(e) {e.preventDefault()},
false);
</script>
<p>When clicking the button below, you should get a prompt asking if you want to unload the document</p>
<form method="get" action="next.html">
<input type="submit" value="Click here">
</form>