We've been styling fonts for years using
ems with this technique: http://www.alistapart.com/articles/howtosizetextincss
But I think its time to change to this method: http://snook.ca/archives/html_and_css/font-size-with-rem
That method uses
rems with a
px fallback. The browser support is ok; see http://caniuse.com/#search=rem
em is hard, especially when you want to change font sizes. Since the font size cascades and all font sizes are relative to the parent, you end up having to recalculate a lot of em values. :-p And there's no easy way to automate that math with Sass since Sass doesn't automatically know what the parent elements font size is.
But with a px fallback, we can easily create a Sass mixin to write px fallback and then rem.