It can't find pleaseWait because it's defined in a theme stylesheet, not in a CSS file that is directly included in a master page for example.
The design issue is probably a bug in Visual Studio 2008. I have that every now and then as well. During early betas this issue has been reported and fixed, but sometimes it seems to reappear.
First, try removing the style in the head of the page:
If that fails, try this:
1. Close Visual Studio
2. Open Visual Studio and close all open files
3. Right-click the Solution or Project and choose Rebuild / Rebuild All
4. Resolve any errors and repeat steps 1 through 3.
5. Open the User Control in design view.
If it still doesn't work, I suggest you report this issue with Microsoft as it is supposed to work.....
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