Einstein's second postulate of special relativity is:

The speed of light takes the same value ($c$) for any observer in an inertial frame of reference.

I know there is a lot of experimental evidence for this statement, but has it every been proven (theoretically) to be the result of another ("deeper"?) theory, that does not take it as a postulate?

Einstein's second postulate of special relativity is:

The speed of light in vacuum takes the same value ($c$) for any observer in an inertial frame of reference.

I know there is a lot of experimental evidence for this statement, but has it every been proven (theoretically) to be the result of another ("deeper"?) theory, that does not take it as a postulate?

From Einstein's second postulate of special relativity:

The speed of light takes the same value ($c$) for any observer in an inertial frame of reference.

I know there is a lot of proof experiment for this statement, but has it every been proven (theoretically) to be the result of another theory (/part of physics), that does not take as a postulate the consistency of the speed of light?

Einstein's second postulate of special relativity is:

The speed of light takes the same value ($c$) for any observer in an inertial frame of reference.

I know there is a lot of experimental evidence for this statement, but has it every been proven (theoretically) to be the result of another ("deeper"?) theory, that does not take it as a postulate?