A few things you might not now about the asp:Panel-control

The DefaultButton property let’s you specify the ID of a button control. Doing this will hook this button to the Enter key whenever any type of input control within the panel is focused.

Setting the GroupingText property results in the following accessible friendly markup using fieldset and legend-elements;

<div id="pnl">
    <fieldset>
        <legend>GroupingText value goes here</legend>
        Panel content is rendered here
    </fieldset>
</div>