Lo stile all'html si deve impostare, ma la sintassi che normalmente si usa e` un'altra:
codice:
<?xml version="1.0" encoding="iso-8859-1"?>
<!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>
html {
	height:100%;
	/* ... */
}
body {
	height:100%;
	margin:0px;
}
.prova {
	display:block;
	background:#000080;
	width:100px;
	min-height:100%;
	height: auto !important; 
	height: 100%;
}
</style>

</head>

<body>


<div class="prova">
prvoa

</div>


</body>
</html>