Typography is a specific skill set, nobody said that it should be 'easy to understand'. Makes sense to whom - to web developer? He is not the intended audience.

Typography is a specific skill set, nobody said that it should be 'easy to understand'. Makes sense to whom - to web developer? He is not the intended audience. If the font size would be specified using only cap height then the work of font designer on specific relationship between negative and positive space and line height would be thrown out. That would be a net negative for the state of typography on Web. Again putting things into fixed size containers that do not adjust based on different font face is not a good practice and it is not what Web meant to be.