As grammar is a description of how language works, it cannot be prescriptive.
Advanced English Dictionary
+adjective 1 (formal) telling people what should be done: prescriptive methods of teaching 2 (linguistics) telling people how a language should be used, rather than describing how it is used: a prescriptive grammar of English Antonym: DESCRIPTIVE 3 (technical) (of rights and institutions) made legal or acceptable because they have existed for a long time: prescriptive powers
Random quote: It is impossible to feel grateful and depressed in the same moment.: Naomi Williams