[aphyr]

Eistein's original formulation of SR required two postulates: that the laws of physics are indistinguishable between any two inertial reference frames, and that the speed of light is constant in all inertial reference frames. Both of these are experimentally motivated.

However, that's not the only way to obtain SR. You can, for example, assume Maxwell's equations (or any of the higher-level electrodynamic field formulations) and postulate #1, and obtain the invariant speed of light as the only valid solution for self-propagating EM waves in free space. I rather prefer this formulation, as the EM field formulation feels more "fundamental" than a fixed speed of light.

You can also derive SR from postulate #1 and some properties of space: isotropy, homogeneity, and memorylessness.