Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<script>
function showMsg()
{
alert("You have changed the size of the browser window!");
}
</script>
</head>

<body onresize="showMsg()">

<p>Try to resize the browser window.</p>

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.