It’s not just difficult to understand, the em height / character shape described by the article are total nonsense — they have zero relationship to the font its

It’s not just difficult to understand, the em height / character shape described by the article are total nonsense — they have zero relationship to the font itself, and no relationship between one font’s eg height and another. Which ultimately means that font sizing is entirely nonsense: the only thing you can say is font size 1 is smaller than font size 2, but it’s impossible to say by how much, or guess the difference, except when talking about one, and only one font. That is, a font with size 16 roughly corresponding to the same size in another font is entirely defined and enforced by nothing at all — it’s purely a “happens to be true”. Thus, the only real way to handle font sizing predictably is to never switch them out, because you’re really dealing with undefined behavior