Doppler effect - the observer starts moving toward the light source:

https://www.youtube.com/watch?v=bg7O4rtlwEE

The frequency measured by the stationary observer is f. Accordingly, the speed of the light pulses relative to him is c=df, where d is the distance between the pulses.

The frequency measured by the moving observer is f'f. Accordingly, the speed of the light pulses relative to him is c'=df'c.

Obviously variable (Newtonian) speed of light.

Note that the wavelength (distance between pulses) is constant. Here this is obvious; in other scenarios the constancy of the wavelength of light is not so obvious but remains true.

If there is a next, Einstein-free version of fundamental physics, Einstein's 1905 nonsensical axiom

"The speed of light is constant"

will be replaced with the correct axiom

"The wavelength of light is constant (for a given emitter)".

I have developed the idea in a series of tweets he https://twitter.com/pentcho_valev

Pentcho Valev