what is the actual proof that the speed of light is constant for all observers?

No.13576010 ViewReplyOriginalReport
I've heard someone say that there is actually no proof and that its just a postulate of special relativity to make it all work. So does that mean we just "accept" this postulate because everything else in special relativity appears to be true? what if thats just a coincidence. What if the speed of light isn't constant but something else and all the theories in special relativity would still be true in that case?

I mean doesn't the law of relative velocities directly contradict the idea of light having a constant speed to all observers?