The RoundedCorners extender applies rounded corners to existing elements. To accomplish this it inserts elements before and after the element that is selected, so the overall height of the element will change slightly.

The control above is initialized with this code:

 <ajaxToolkit:RoundedCornersExtender ID="rce" runat="server"
    TargetControlID="Panel1" 
    Radius="6" />

• TargetControlID - The ID of the button or link for this extender to operate on
• Radius - The radius of the corners (and height of the added area).  Default is 5.
• Color - The background color of the rounded area an corners.  By default this picks up the background color of the panel that it is attached to.