Q: Why do we call quantum physics an ab initio model? Why do we call classical mechanics an ab initio model?
A: Because it only relies on the basic equations without any additional assumptions.
Q: Is classical electromagnetism an an ab initio model (and if it is one, what are its first principles)?
A: Yes, the Maxwell equations.
wow, ok yes .. that sounds right. I thought since we use classical mechanics in electricity, magnetisim and in particular waves then EM should be not ab initio, but may be I am mistaken .. I think you are right