One of the problems with using "em" as font sizes is that they cascade, so you are forever writing rules that select nested elements to reset them back to 1em; CSS3 now has rem ("root em") to prevent that issue. Just declare a px fallback and you can use it today. Snook shows us they way.
Many web designers and developers are familiar with the CSS remlength unit. But, you may not know that it has a couple of handy alternate uses. In this post, I’ll describe how to use the CSS rem unit to scale specific page elements while leaving others unaffected. I’ll also discuss how to use it as a replacement for the under-supported vw (viewporth width) unit.
Many (especially decorative) web fonts come only in a few selected weights and styles. It's smart to load as few as possible since they can be quite heavy, but also make sure you only use them in the weights/styles you load otherwise the browser will fake it for you and muddy it up.