codice:
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    <div id="top">TOP DELLA PAGINA</div>

     ......... Lungo contenuto...............

    <a href="#top">TORNA SU</a>
    </div>
    </form>
</body>
</html>