It goes back to Internet Explorer 3.0, but EMs are still the better way to define font-sizes. It takes a small learning curve to get the hang of it, but when it comes to quick changes in overall font-size, this is the way to go. Default font-size for most common browsers is 16 pixels. There are only three articles you need to read to tame that size with relative ems.
1em = 12pt = 16px = 100%
Useful reads
- More fun with font sizes (Meyer, 1999)
- How to size text using ems (Clagnut, 2004)
- Care with font size (W3 schools, 2009)
The EM formula
Child pixels / Parent pixels = Size in EM
Not good at calculating? Use the EM calculator
Still targeting Internet Explorer 6?
While Internet Explorer handles EM, it does so only when the base font is set to a percentage!
body { font-size: 62.5%; }
body wrapper { font-size: 1em; }