My Buttons look like this in the code:

     <div id="buttonBar" class="buttonBar">

           <button id="toggleTreeButton" runat="server"
onclick="ToggleTree()" title="Toggle tree pane">

                 <img src="/images/toggle.gif" alt="Toggle tree pane" />

           </button>

           <button id="moveSplitterLeftButton" runat="server"
onclick="MoveSplitter('left')" title="Move splitter left">

                 <img alt="Move splitter left" src="/images/moveleft.gif"
/>

           </button>

           <button id="moveSplitterRightButton" runat="server"
onclick="MoveSplitter('right')" title="Move splitter right">

                 <img alt="Move splitter right" src="/images/moveright.gif"
/>

           </button>

           <button id="printableButton" onclick="ShowPrintable()"
title="Show printable version">

                 <img alt="Show printable version"
src="/images/printable.gif" />

           </button>

     </div>

You were absolutely right. I changed the type to type="button" and
everything worked well.

Thanks!