The standard is uselessly incomplete and vague. There are places where MUSL implements a broad set of GLIBC extensions in order to actually be useful. However,

The standard is uselessly incomplete and vague. There are places where MUSL implements a broad set of GLIBC extensions in order to actually be useful. However, it does not indicate that in any way, and sometimes violates the conditions that GLIBC documents. This requires workarounds. There are places where MUSL implements a standard interface in a particular way. If you're lucky, this "just" means giving up on performance if you don't know you're using MUSL. Sometimes MUSL implements its own ABI-incompatible extensions. The time64 transition, for example, is a huge mess regardless, but musl provides no blessed way to figure out what's going on. The only reason it's not an even bigger disaster is that almost nobody uses musl.