html, body{height:100%; margin:0; padding:0;}
.content{ width:100%; margin:0 auto; border:0px solid #adadad;}
#tasto_top{background:url(img/up1.png); width:45px; height:46px; display: none; z-index:9; position:fixed; top:100%; 
 margin-top:-50px; right:5%; margin-left:10px; opacity:.8; text-decoration:none; }
#tasto_top:hover{background:url(img/up2.png); width:45px; height:46px; display: none; z-index:9; position:fixed; top:100%; 
 margin-top:-50px; right:5%; margin-left:10px; opacity:.8; text-decoration:none; transition-duration: 1s;}