Dozens of experiments have been performed with light rays, beginning from Fizeau, in order to establish the local constancy of the speed of light and verify how speeds sum up.
It was verified that the speed of light is independent on the speed of its source, otherwise it would be not hard to determine an absolute speed of the platform of emission.
From the verified independence of the speed of light on its source, one might expect also the independence of its direction on the normal component of the source.
Lorentz Transformations predict light tilting.
That is the basic principle on which Einstein's clock is based.
Synchrotron light is said to be based on same principle
http://www.lnf.infn.it/edu/stagelnf/2015/inspyre/slides/19Feb/Balerna_SynchrotronLight.pdf