For what it is worth, Kenneth G. Wilson identifies his work The Columbia Guide to Standard American English as both prescriptive and descriptive, accepting that there are, indeed, prescriptive rules, but that they do not cover all usage questions. Perhaps he includes genitive its under the prescriptive rules when he says:
I actually consider Wilson to be descriptive rather than prescriptive, in the fashion of Merriam-Webster's Dictionary of English Usage. When it comes down to it, every dialect has rules that must be obeyed in order for the speaker or writer not be thought of as odd. Even when English spelling had no standard rules, as we think of as standard today, someone who wrote toag for the word goat would have been seen as being in error. It just so happens that genitive its as the acceptable spelling is a rule agreed upon by both sides of the usage debate.its, it's
The genitive of the pronoun it is its, without an apostrophe; the contraction of it is is spelled it's, with an apostrophe. Most errors involve using the contraction when the genitive is intended; these are usually the errors of inattention or carelessness, but they're often judged as though ignorance caused them, so inspect carefully what you write.