Codice PHP:
<!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>test</title>
<style type="text/css">
html {
    height:100%;
}
body {
    margin:0px;
    height:100%;
}
.main-body{
    width:780px;
    margin:0px auto;
    height:100%;
    background:#cccccc;
}
</style>

</head>
<body>
    <div class="main-body">
        asd
    </div>
</body>
</html>