`em` vs. `rem` for sizing... discuss?

@cferdinandi `rem` for pretty much all the fonts, `em` for the relationships between those fonts.

I still use `px` for spacing of none-typographic things, but I know I don't want to.

@devolute @cferdinandi this is my view as well. Font size in `rem`, spacing in `em`, and `px` for flourishes like `border-radius`