Currently the mechanics of the smallest things we know are subatomic particles. If we ask why some physical phenomenon occur in our surroundings, and then keep asking why, then we will eventually reach to the sub-atomic world where if we asked why, then we will get an answer which will be based on some assumption which are not yet violated or least violated. So the smallest world till this day where we will get our answer to everything is subatomic world. So anything can be modelled with a good model of subatomic world. Phenomena of this subatomic world are explained by quantum physics, so we call quantum physics as ab initio modelling. Some phenomena can fully be explained by classical mechanics without going into quantum mechanics. In such cases classical mechanics can be termed as ab initio. For example, explaining the phenomena occurring on a large scale like motion of planets can be explained by classical mechanics alone.
Classical electromagnetism may not be considered as ab initio method as it assumes that quantum effects are not there. But if we try to explain phenomena with low field strength and small distance it leads us to the quantum physics.