Oh, and yes - we use fixed pixel font sizes.
Relative font-sizes are a nightmare - different browsers inherit differently in the HTML structure and base things on different default sizes.
This causes problems if you use percentage sizing, or ems, or relative named sizes - medium, small, smaller. Compare small with 12px fixed size - wouldn't you think 12px was more of a medium size for text? It used to be, back in the days of 72ppi. But you have to use 16 pixels if your monitor is 96ppi to get the same physical size as 12px on a 72ppi screen.
We currently use 11px for body text on this site.