Massless particle (such as a photon) cannot have a frame of reference. But the electromagnetic field does have mass; does it have a frame of reference? If so, if one takes a charge as "setting up" its own EMF, then the field's frame of reference is the charges, so the distinction doesn't matter. But if we take the EMF to pervade all of space, and charges merely to alter the value of the field at any given location, then it makes sense to ask about the frame of reference of the field per se. However the EMF varies from point to point; how could a volume of the EMF, which is not homogenous, have a single frame of reference?