CSS is hard to unit-test because nearly all the places where it can be messed up are detected by a human who says "Hey, this looks really ugly/hard to read/misorganized", a category of problems that is generally hard to write automated tests for. I don't think it's a fault in the language, but the application domain.

C++ is also hard to unit-test, but in that case I agree that it really is part of the language. I enjoy working with C++ and use it for some of my own projects, but if I'm being honest I have to admit that its near-total lack of reflectivity and numerous odd potholes and tripwires makes it much less convenient to do certain sorts of things with it, in-language automated testing being a prominent one of those.

I'm optimistic about Vala, an in-development C#/Javaish language that compiles to Glib-using C and supports native (but language-mediated) access to C libraries, so you get all the performance and platform-specific benefits of working in C/C++, but with modern language features and a noticeable lack of C++'s slowly expanding layers of cruft.