"Attention to correctness
musl was the first Linux libc to have mutexes safe to use inside reference-counted objects, the first to have condvars where newly-arrived waiters can't steal wake events from previous waiters, and the first to have working thread cancellation without race conditions producing resource-leak or double-close. All of these are requirements of the specification that were ignored by other implementations, and getting them right was a consequence of careful reading of those specifications.
musl's entire development history has been a process of reading specifications, seeking clarifications when corner cases aren't adequately covered, and proceeding with extreme caution when implementing functionality that's underspecified."