CSSCop makes it easy to adhere to best practices for writing stylesheets. It catches common errors that affect browser compatibility and much more. You can think of it as FxCop for stylesheets.
CSSCop for Visual Studio tries to help us write better CSS, by making sure we remember all the right vendor specific properties or that you cannot set the margin on inline HTML elements etc.
CSSCop is developed by Mads Kristensen - Program Manager at the Microsoft Web Platform Team and founder of BlogEngine.NET.