Ok, dovrebbe andare:
codice:
<!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=utf-8" />
<title>Untitled Document</title>
<style>
#footer{
width:980px;
height:213px;
margin-left:auto;
margin-right:auto;
background: transparent url("../images/footer.jpg") top left no-repeat;
border-bottom:20px solid white;
border-top:1 px solid transparent;
background-color: #f00; /* l'ho messo solo per fare il test, rimuovilo */
position: relative;
}
#by{
width:300px;
height:auto;
border:1px dotted pink;
background-color:white;
/*margin-left:auto;
margin-right:auto;
margin-top:3em;*/
position: absolute;
bottom: 0px;
left: 340px; /* = (980px - 300px) / 2 */ }
</style>
</head>
<body>
<div id="footer">
test
<div id="by">
altro test
</div>
</div>
</body>
</html>