But, Einstein's great insight was that this is in fact not the case for light.
But I think something is missing there.
I was questioning the meaning of "constant speed".
As I see it, the constancy of the speed of light is used as a sort of standardizing principle, so as to find ways of translating measuring scales between different inertial frames. It is unlikely that the OP was asking about standardizing principles. So I wanted to know what he thought he was asking about.
Fundamentalism - the anti-American, anti-Christian branch of American Christianity