Why would it matter? The bytes aren't inline, this is just a struct with two word-sized fields.
A possible tiny advantage for this layout is that a pointer to this struct can be used as a pointer to a pointer-to-bytes, without having to adjust it. Although i'm not sure that's not undefined behaviour.
I don't think that's undefined behavior. That's how C's limited form of polymorphism is utilized. For example, many data structures behind dynamic languages are implemented in this way. A concrete example would be Python's PyObject which share PyObject_HEAD.
