CSS rollover (single image) with ASP.Net HyperLink

Hi all,
I've been using a html link and some css as below to create a rollover which changes the background position when hovered over. But I can't get this to work when I change to a ASP:HyperLink which I need to use with one of the AJAX Extenders. Can anyone advise?
 
<a id="feedbackButton" runat="server" href="http://forums.asp.net/AddPost.aspx?ForumID=18#"></a>
 
a.feedbackButton
{
    width: 75px;
    height: 25px;
    display: block;
    padding: 0;
    margin: 0;
    background: url("../App_Images/button_feedback.png") 0 0 no-repeat;
    margin: 0 0 0 40px;
    bottom: 10px;
}
a:hover.feedbackButton
{
    background-position: 0 -25px;
}
 
Thanks
Denis Go to the complete details ...