Since obsolete unterbrowser users must suffer, rem FTW.

What's REALLY missing in modern CSS3 standard is the ability to reassign CSS root to any other element than <html>, according to font properties of which all nested rems would be calculated. So, for now, we have to stick to assigning base font sizes to <html>, not even to <body>.

My microframework (999.css - http://999.surge.sh) uses pixels to assign base font size to <html>, rem for <body>, ems for headings and rems for all other nested typography.