实现半透明效果

1)<style type="text/css">
     .modalBackground
     {
        background-color:Gray;
        filter: alpha(opacity=50);
        opacity: 0.5;
     } 
     </style>

2)

<div >
        <iframe style="Z-INDEX:-1;WIDTH:100%;POSITION:absolute;HEIGHT:100%">
        </iframe>

        <table id ="imgTable" runat="server" style=" width:100%; height:100%;">
        <tr><td align="center" valign="middle"><asp:Image ID="Image1" runat="server" ImageUrl="http://www.cnblogs.com/images/pleasewait.gif" /></td></tr>
        </table>
    </div>

<input type="button" />