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.
Both prescriptive standards and descriptive standards have their uses. If POSIX is a prescriptive standard, then maybe another standard should exist that is descriptive.
Keep in mind that the Web standard eventually became prescriptive because descriptive standards failed to catch up. Likewise it can be argued that descriptive standards for the common OS interface are no longer usable.
To be crass, description is only useful for existing things and prescription hinders making innovative things. I think social forces make it natural that standards are treated both descriptively and prescriptively, and that too leads to angst. Case in point, POSIX was once more descriptive, but then people wanted backwards compatibility for existing and new OSes, which made it more prescriptive. The takeaway is that ad-hoc things become permanent once they are too difficult to remove, and then people are sad. Nothing is immune, so just make reasonable attempts for the standard and the culture to harmonize for a specific purpose.
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