1) w3schools is filled with errors and omissions. Better tutorials can be found at howtocreate.co.uk
2) You have no DOCTYPE declaration, and you're using XHTML syntax.
2.1) IE doesn't support true, see webdevout.net/articles/beware-of-xhtml for more information
3) You need to encapsulate the within a element as well as another block-level element as per the specification
See below for a proper HTML5 document. Notice the location and syntax
<!DOCTYPE html>
<html>
<head>
<title>Hello!</title>
<script>
function confirmBox() {
var ret = confirm('Some Text');
/*
Note the 3 equal signs. This is a strict comparison operator, to check both the 'value' as well as the type. see https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference/Operators for more
*/
if(ret === true) {
alert('Alert box for "Okay" value');
}
else if(ret === false) {
alert('Alert box for "Cancel" value');
}
}
window.onload = function() {
// Execute the confirmBox function once the 'button' is pressed.
document.getElementById('confirmBox').onclick = confirmBox;
}
</script>
</head>
<body>
<form>
<p>
<input type="button" id='confirmBox' value="Show Confirm Box">
</p>
</form>
</body>
</html>