IDesignTimeHtmlProvider Interface
You can implement this interface to have a different HTML rendition for your Web parts and controls in SharePoint Designer than what you see in the browser. Well, if that’s what you are looking for, IDesignTimeHtmlProvider interface is definitely your friend!
As you can see in the pictures below , my simple Web part renders two different type of messages at run-time and design-time. For more information see my notes in the community content of this page.
Browser:
SharePoint Desginer:
Categories: MOSS 2007