[gpderetta]

"valid program" is not really a term that is used in the standard (I only count one normative usage). What the standard does say is:

"A conforming implementation executing a well-formed program shall produce the same observable behavior as one of the possible executions of the corresponding instance of the abstract machine with the same program and the same input. However, if any such execution contains an undefined operation, this document places no requirement on the implementation executing that program with that input (not even with regard to operations preceding the first undefined operation)."

I.e. a program the contains UB is undefined.

Of course, as you observer, an implementation can go beyond the standard and extend the abstract machine to give defined semantics to those undefined operations.

That's still different from implementation defined behaviour, where a conforming implementation must give defined semantics.