Sure, this isn't meant to make it impossible to shoot yourself in the foot design-wise if you intermingle fonts. It's more of a layout concern. If I change th

Sure, this isn't meant to make it impossible to shoot yourself in the foot design-wise if you intermingle fonts. It's more of a layout concern. If I change the font in an app or website, it _always_ leads to tweaking all the font sizes and adjusting vertical padding on things like buttons. All to get the baseline and vertical height at roughly the same spots between fonts. The author's proposal of a "cap height" (I don't like that term, though) would make that tweaking a thing of the past.