codice:
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>Documento senza titolo</title>
<script language="javascript">            
function f(page) {                 
var w = 600;                 
var h = 350;                 
var t = Math.floor((screen.height-h)/2);                 
var l = Math.floor((screen.width-w)/2);                 
var stile = "top="+t+", left="+l+", width="+w+", height="+h+", status=no, menubar=no, toolbar=no scrollbars=no";                 
window.open(page,'log', stile);            
}
</script>
</head>

<body>

<form action="l.php" method="post" target="log" onsubmit="f('l.php')">
<fieldset>
<input name="user" type="text" placeholder="username" >
<input type='submit' name="Accedi" value="Accedi" >
</fieldset>
</form>
</body>
</html>
esempio online