CSS滤镜的背景图片里包含的控件不能获得焦点

[antsing]

CSS:
.shadow
{
filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src="images/tbLogin.png" ,sizingMethod='crop' );

}
html:
<table class="shadow" border="0" style="height: 236px; width: 540px; bottom: 0px; right: 0px;" cellspacing="0" cellpadding="0">
<tr>
<td height="25" style="text-align: right; width: 167px; color: black;
position: relative;top: -5px; left: 0px;">
用户代号：
</td>
<td height="25" align="left" style="width: 159px;">
<asp:TextBox TabIndex="0" autocomplete="off" runat="server" Width="150px" ID="txtUser" Text="admin" />
</td> </tr>
</table>