A lot of people are saying that we can’t use cap height because of non-Latin characters and that was also my first thought. But we could say that every charact

A lot of people are saying that we can’t use cap height because of non-Latin characters and that was also my first thought. But we could say that every character is defined relative to the cap height of “M” and then it is up to the font designer to choose how big an あ or a ㅕ is relative to the M. That would however not be much of a change compared to an em, which basically just says “how big is your line compared to the tallest line available?” Unlike the author, I haven’t created a successful font, but my feeling is that it is better to have things defined relative to a max height, than have things defined relative to “a height”.