Hopefully nothing, posix is, or at least it should be, a descriptive standard. This is why posix is so terrible, and why posix is so great.

The way I feel posix, and other descriptive standards work best is when they describe what every one is already doing. This is opposed to prescriptive standards which try focus on how the "correct" way to do somthing, prescriptive standards tend to be over engineered and may or may not actually work.

see also: descriptive and prescriptive dictionaries. http://www.englishplus.com/news/news1100.htm