I use this piece of code to inform user to fill some field in this way:
ob_start();
$msg = $_SESSION['ErrorUser'];
if($msg !=""){
echo '<script type="text/javascript">alert(" ' . $msg . '");</script>';
}
//echo $msg;
$_SESSION['ErrorUser'] = "";
ob_end_flush();
This works in Internet explorer while in FF and Chrome. The msg is displayed, but I have nothing in background, just a part of my page is showed (behind my Message alert).