codice:
<!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=utf-8" />
<title>Documento senza titolo</title>
<style type="text/css">
<!--
#div_centrato {
	font-family: Verdana, Geneva, sans-serif;
	font-size: 12px;
	font-style: normal;
	line-height: normal;
	font-weight: bold;
	color: #FFF;
	text-decoration: none;
	background-color: #00F;
	text-align: justify;
	text-indent: 35;
	padding: 0px;
	height: 500px;
	width: 500px;
	margin-top: -250px;
	margin-left: -250px;
	border: 3px solid #000;
	position: absolute;
	left: 50%;
	top: 50%;
}
-->
</style>
</head>

<body>
<div id="div_centrato">Inserire qui il contenuto per del box!</div>
</body>
</html>