Postel's Law was one of those Great Mistakes of computing, alongside null pointers, fork(2), and well, C in general. Conform to the spec or be in error. If you allow for sloppiness, you create a problem because different implementations will tolerate different kinds of sloppiness, yielding incompatibilities and horrors like "quirks mode".

XHTML tried to rein this in but by then the cat was out of the bag, and every Tom, Dick, and Mary who was trying to learn HTML was used to the mire tolerant behavior.