vabbè..ho letto il + recente..solo k non funziona!
codice:
HTML 
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento senza titolo</title>
<link href="stile.css" rel="stylesheet" type="text/css" />
<link rel="stylesheet" type="text/css" href="niftyCorners.css">
<link rel="stylesheet" type="text/css" href="niftyPrint.css" media="print">
<script type="text/javascript" src="nifty.js"></script> 
<script type="text/javascript">
window.onload=function(){
if(!NiftyCheck())
    return;
Rounded("div.nifty","all","#00FFFF","#D4DDFF","smooth");
}
</script> 
</head>

<body><div class="nifty">
nifty_prova</div>
</body>
</html>

CSS 
div.nifty{background:#0099FF;
width:150px;
height:100px;}