It is essential and nontrivial, but it's also the extremely bare minimum.

You cannot write portable code without platform-specific and even environment-specific adaptations, like handling the presence of certain headers (looking at you, stdint.h and stddef.h), and let's not even start about interacting with the OS in any way.

There may be platforms that are not conforming to the C standard. But I doubt those then have comprehensive implementations of other languages either.