Microsoft not supporting C99 is a huge issue because the world of open-source C projects that are supported on both Unix flavors and Windows can not integrate C99 features into their core because Windows support means building with MSVC. It might not be a big deal for one individual project, but you're literally talking about the entire world of open-source C projects being restricted by their decision.

In this case (open source projects using C99) they can use mingw.

An ironclad rule of platform-specific development: You use the compiler that everyone else is using. On Windows, that means Visual Studio.

I'm really happy for platform-specific development, and I'm gonna let it finish, but cross-platform development is the most cross-platform development of all time.. of all time!