Fact:
There is no definite proof that the speed of light is constant in all reference frames. This is merely an axiom (a geuss) made up by Einstein.

Just because our technological tools and experimental designs are currently too weak to detect differences in the speed of light across different reference frames, it doesn't mean they aren't there. Relativity is bunk and everyone who dogmatically believes in it is as much of a brainlet as Einstein.