Yahoo!'s UI Library offer a CSS reset which makes all elements in all browsers reset to zero. Thankfully Yahoo! have made the reset publicly available.
By giving the body tag a font size of 62.5%:
all subsequent elements can be styled like this:
Now, due to resetting the CSS and giving the body a font-size of 62.5%, all em values are related to pixel values in that 2em = 20px, 1.5em = 15px and so on.
There you have consistent cross-browser text styling.