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Usually it's not the Automata theory parts of Regex that people find hard. It's the syntax.

Understand the Box Model and you'll understand all of CSS.

Hogwash. Where in the box model are the precedence rules for selectors defined? How does the box model help with page layout and positioning? Animation? Transformations? Pagination?

Sure, the box model is a prerequisite to understand CSS properly. But if you think the box model covers all aspects, you really don't know very much about CSS.

> there’s no underlying mathematical theory that explains css

There's a well-defined algorithm (well, set of algorithms) that explain it, which you can learn if you so choose.