These are the two principles of Special Relativity:
The laws of physics are applied identically in all Inertial frames of reference.
The speed of of light is the same in all frames of reference.
Did Einstein come up with the first one or did he just used what Galileo said about motion in 1600? Galileo was the first to state that velocity is relative. And since velocity is relative I don't see what else includes the above-mentioned first principle of Relativity. All Classical Physics up 1900 included that and so does modern Relativistic Mechanics.