This website never ventures outside Newtonian mechanics. It's not that I haven't read up on relativity or quantum physics, but it never resonated with me. Clearly I'm an engineer at heart, not a scientist; and for everyday engineering, classical mechanics is quite sufficient. You could say I'm a keen amateur from the late 19th century.
You will find some material which uses linear algebra, a field which only became popular in the second half of the twentieth century I think. But that's about it. Even in aerodynamics I'm quite subsonic.
Everything on this website should be accessible to anyone with a basis in high school math and physics.
Having said that, not all of the material is trivial. I find that most of my interests tend to come from the last "extra" chapter of old textbooks, or from other quirks. But if you are a quirk yourself, this should not worry you.