if you are not able to open css files in Visual Studio 2010 because of any reason and keep getting this error message:
“visual studio 2010 the operation could not be completed css”
here is what you need to do:
Click on the Tools Menu => Extension Manager => Select Online Extensions => Search for “Web Standards Update for Microsoft Visual Studio 2010 sp1” and install the extension.
if you have already installed the extension, run the setup and repair it.
in my case it happened after I installed MVC4 beta. and althogh I had already had the extension installed it was not opening CSS files and repairing the extension fixed it.