It's also important to know which rules are "hard", i.e. implemented or enforced by the first three (or implemented by the program, but would be a complete nons

It's also important to know which rules are "hard", i.e. implemented or enforced by the first three (or implemented by the program, but would be a complete nonstarter to break) and which rules are just "conventions", like the "short options/long options" rules in POSIX. In particular, those can change between OSes: On all linuxes, programs already get a pre-parsed array of command line args on start, so parsing and quoting behavior depend on the shell, not individual programs. On Windows, programs get passed the entire command line invocation as a string and have to do the parsing themselves. While all "well-behaved" programs let the libc do this, it's perfectly possible for a program to break the rules here.