ImageButton With Mouse Event Handlers Considered Not Well Formed

ASP.NET says that the following ImageButton server tag is not well formed:

Solution 2:

    <asp:ImageButton runat="server" ID="imgAdd" src="Images/add_plus_1.gif"
    onmouseout="this.src='Images/add_plus_1.gif'"
    onmouseover="this.src='Images/add_plus_2.gif'"
    onmousedown="this.src='Images/add_plus_3.gif'"
    CauseValidation="false" style="cursor: pointer"
    OnClientClick="Javascript:SL_AddBuilding()" />