Web Forms :: ImageButton Produces Invalid HTML: Border="0"?

I'm getting an error out of WC3's HTML validator.

This is the error:

Line 102, Column 346: there is no attribute "border"

..., "", true, "", "", false, false))" border="0" />

You have used the attribute named above in your document, but the document type you are using does not support that attribute for this element. This error is often caused by incorrect use of the "Strict" document type with a document that uses frames (e.g. you must use the "Transitional" document type to get the "target" attribute), or by using vendor proprietary extensions such as "marginheight" (this is usually fixed by using CSS to achieve the desired effect instead).

This error may also result if the element itself is not supported in the document type you are using, as an undefined element will have no supported attributes; in this case, see the element-undefined error message for further information.

My client wants my pages to validate.

This is my markup:

<asp:ImageButton
ID="cmdTabMyPlanner"
CssClass="NavigationTab"
ImageUrl="images/nav-2.jpg"
runat="server"
/>

And this is the HTML it produces:

<input type="image" name="ctl00$cphMenu$cmdTabMyPlanner" id="ctl00_cphMenu_cmdTabMyPlanner" class="NavigationTab" src="images/nav-2.jpg" onclick="javascript:WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions("ctl00$cphMenu$cmdTabMyPlanner",
"", true, "", "", false, false))" border="0" />