I'm sure grandparent knows that. The problem with cascadingly using em is that you cannot easily deduce the derived em-value on the nth level (e.g. header > nav

I'm sure grandparent knows that. The problem with cascadingly using em is that you cannot easily deduce the derived em-value on the nth level (e.g. header > nav > div > a, each with their own em-based font-size; good luck reasoning about a's em-value).