The speed of light is, but it does not have anything to do with relativity. Those concepts are connected by the speed of light in vacuum, yes, but there is no

The speed of light is, but it does not have anything to do with relativity. Those concepts are connected by the speed of light in vacuum, yes, but there is no logical reason for that. This was an educated guess, and was confirmed by observation it's not a conclusion. Einstein started to formulate special relativity theory starting from an assumption that a "ball of light" won't change its shape for non-inertial (no accelerarion) observers regardless of their respective speed. I.e. constant speed of light was the reason not a result of this theory. (Duh. This got lenghty.)