On https://www.netbsd.org/docs/ There is a "Single UNIX Specification" link: http://www.unix.org/online.html
This should probably be supplemented by the somewhat more modern and useful "POSIX.1": https://pubs.opengroup.org/onlinepubs/9699919799/ This is simply a more modern version of what's already linked, and far more importantly does *not* require registering or logging in to read the spec.
