In design mode, this happens to me all the time..
I pretty much strickly work right from the HTML markup and code by hand.
I dont like WYUSIWUG editors.
However, once you run the site, it should still work with out to many issues..
I dont know if there any good way around the errors inside the visual layout..
You may be able to add the dll references, but I don't know.
When I test, I just run visual studio and preview with a browser but code everything via markup..