Are you aware of HTML 5? Fun fact about it: there's zero leniency in it. Instead, it specifies a precise semantics (in terms of parse tree) for every byte sequence. Your parser either produces correct output or is wrong. This is the logical end point of being lenient in what you accept - eventually you just standardize everything so there is no room for an implementation to differ on.

The only difference between that and not being lenient in the first place is a whole lot more complex logic in the specification.