From the website:
If you play d20 . . . this 48-page hardback book shows how it should be played. If you don't play d20, feel free to inflict munchkinism on some other system. The Munchkin RPG will jazz up your sessions as your characters rocket up in power! This book includes munchkin classes, races, spells, and tips and techniques designed to terrify even a hardened GM!
This is d20 the way it SHOULD be played. Munchkin out!