Workaround: Empty Design View for User Controls
I've noticed quite a few people run into the issue "Why is design mode empty for my web user control (ASCX)?". It's not obvious what might be causing this bug, so I thought I'd offer an explanation and a couple workarounds. Problem You have a user control that has a "link" tag to an external style sheet or a "style" tag with local styles at the top of the page. Immediately below the style, you have a web control (such as a Wizard control) that contains the bulk of the content. You attempt to edit this user control in design view and all you see is an empty page. Here's a trivial example: Explanation Design view was originally created to handle full HTML pages where the "body" and "head"
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Read the complete post at http://blogs.msdn.com/webdevtools/archive/2008/04/27/workaround-empty-design-view-for-user-controls.aspx