CSS-Tricks has written a good overview of the different available font sizing options within CSS including their limitations and browser peculiarities. At MarketNet our standard practice is to specify the font-size on the body tag using pixels for precision and then em‘s through the rest of the stylesheet. This allows us to easily change the size of all type on the site simply by modifying the value set on the body tag, possibly using a JavaScript font-resizer.

Link: px – em – % – pt – keyword