prova così:
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>sempre centrato</title>
<style type="text/css">
<!--
body {
	margin: 0px;
}
-->
</style>
</head>
<body>
<table>
		<tr>
				<td width="300" height="300" align="center" valign="middle">testo sempre centrato</td>
		</tr>
</table>



</body>
</html>
nb
ho cambiato il doctype da xhtml1.0 ad html4.01

ciak