For Quantum Physics, there is a large number of phenomena in nature that can be well described and reproduces by quantum physics black body radiation for example. The same applies for classical mechanics as we can use it to accurately simulate movements from Brownian motion of smoke to rockets.
Although Electromagnetism well describes a range of phenomena related to one topic and we can can use it for example to simulate our smart phones, it is based on classical mechanics, waves in particular. Therefore, we can not describe it as an ab initio model.
I refer to Simon van der Vennet and Jan Claeys’ answers. Classical Mechanics are what I mentioned based on Newton laws. Quantum Mechanics are what I mentioned based on Schroedinger equation.
For Classical EM I was mistaken. It is ab initio model based on Maxwell’s equations.