So why not just use px?
EDIT, answering my own question: "font-sizes and spacial relationships can be responsively sized based on a single font-size property".
So how does this work? Do you just set html font-size 62.5% for "normal" font-sizes and then change that with media queries in order to increase size for mobile etc? Is this best practice for font-sizes nowadays?
What about em? When are you supposed to use that?