(Law) A given ray of light propagates at the same c ≈ 300'000'000 m/s speed relative to every inertial frame.
In point-like terms, this means that the tip of the ray of light is moving at c in every inertial frame.
(Corollary) The speed of light in an inertial frame is independent of the state of motion of the entity that emits it.
It's important to emphasize here that the means of measurement, i.e. coordinate time and distance, were defined based on symmetry considerations backed by the principle of relativity, without making any use of the physical properties of light. And although the above law is surprising, the fact that it's nevertheless in line with the principle of relativity corroborates the same.