So that the vertical rhythm can be maintained in relation to the typographic scale in use across differently sized text, and so that the browser font size setti

So that the vertical rhythm can be maintained in relation to the typographic scale in use across differently sized text, and so that the browser font size settings are applied appropriately but so that neither they nor the degree of typographic scale in use (font size) affect the horizontal layout differently in different elements. Of course, one might want to deviate from that rule as appropriate to build layouts that do partially respond to typographic scale horizontally, but in my experience, that is very rarely good-looking and tends to break layouts.