RadHtmlPlaceHolder in silver light

Well what I meant is in the aspx page which hosts your silverlight application set the windowless property to true.

Open the default.aspx page and you must be able to find what I said. If at all you are hosting the silverlight app in the .html page add the following tag <param name="windowless" value="true"/>

use the control in an aspx page. You can then set the windowless="true" in the same way you set other properties in asp.net.