While the question is quite old, indeed the situation has changed. In 2022 a standard was published, and there are some implementations on GitHub and elsewhere. If interested, I created a minimalistic library for 8-bit AVR in github, that aims at including "useful comments" describing the issues and the inner working of Posit. One key is indeed that the "expressiveness" of a number is limited to 2^size (in bits) so any 8-bit representation of a number can only express 256 different values (maximum). Variations in the representation can only choose different numbers they express exactly, the rest has to be "rounded" (or "overflowed")