Réponse acceptée !
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Document sans nom</title>
<script type='text/javascript'>
function loadi()
{ alert(document.getElementById("menu_dep").style.left);
document.getElementById("menu_dep").style.left = '100px';
alert(document.getElementById("menu_dep").style.left);
}
</script>
</head>
<body>
<div id="menu_dep" style="left:200px;
position:absolute;">blabla</div><a href="javascript:void(loadi());">go</a>
</div>
</body>
</html>