The font size method is a little bit ambitious, sites I've worked on will always have caveats.

The margin application though is very clever and the right way to go about it, even in complex designs you'll be able to apply this to most components (maybe not using the body selector though).

These aren't used more often because many developers aren't very familiar with the complete power that CSS has to offer, take BEM for example it completely eradicates the most powerful feature that CSS has to offer.