[drysart]

That's a truism in general. If you're liberal in what you accept, then the allowances you make effectively become part of your protocol specification; and if you hope for interoperability, then everyone has to be follow the same protocol specification which now has to include all of those unofficial allowances you (and other implementors) have paved the road to hell with. If that's not the case, then you don't really have compatible services, you just have services that coincidentally happen to work the same way sometimes, and fail other times in possibly spectacular ways.

I have always been a proponent for the exact opposite of Postel's law: If it's important for a service to be accommodating in what it accepts, then those accommodations should be explicit in the written spec. Services MUST NOT be liberal in what they accept; they should start from the position of accepting nothing at all, and then only begrudgingly accept inputs the spec tells them they have to, and never more than that.

HTML eventually found its way there after wandering blindly in the wilderness for a decade and dragging all of us behind it kicking and screaming the entire time; but at least it got there in the end.