It builds under Mingw32 as well, and at least an earlier version built with VC++ without modification (I don't have access to a windows machine right now to verify). It also builds under Plan 9, which uses a very old and basic C compiler very far from C99.

Some of the optional extension libraries like networking may not work out of the box, but TinyScheme doesn't have these anyway.

Basically, Chibi was designed as a better TinyScheme. It's just as small, can also easily be included statically without a library, has a nicer (optional) FFI, various features like full Unicode, full numeric tower and hygienic macros (all optional), and is an order of magnitude faster.

The Microsoft compiler supports C99 since version 2013.

Partially, mostly in the library area. It still has some troubles with actual C99 code:

http://bellecrazysnail.wordpress.com/2013/11/14/some-thought...

There's also some weird double personality issues going on with their compilers in MSVC2013, for example they support designated initializers in plain C but not in C++. So you get partial C99 inside pure C or C89 (MS flavored) under C++.

I forgot where I read the quote, but for a decade Microsoft saw the plain C compiler as tool exclusively meant for Windows development. While gcc and later clang adopted and embraced new C idioms, like the struct syntax I mention, they kept their plain C support frozen in time.

I also found weird behavior in MSVC when pushing the C preprocessor VERY hard (nested variadic macros. That would require an entire blog post for itself...)