If the specification is lax, it's because the expected behavior is different across platforms. C (and C++) has to support platforms where bytes are more than 8 bits, where floats are non-IEEE, and (until recently) where signed integers are not 2's complement. If you want a specific behavior and don't care about portability all you have to do is read the ABI spec alongside the standard.
You have wholly missed the point of the warning. It does not matter what the spec says when the instructions fail to implement it, sometimes, on machines other than yours.
