testato in firefox, IE6 IE5:
<!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>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<style type="text/css">
<!--
body , html { height:100%}
#u {height:40%; background-color:#FF0000}
p {line-height:100%}
-->
</style>
</head>

<body>
<div id="u">

</p></div>
</body>
</html>

ciao paolo