The em is a unit meaning “The size of a capital M”.
In particular this is relative, so for nested elements it means “The size of a capital M in my parent element.”
What do you think happens if you have the following piece of css (which we did)?
font-size : 0.96em;
Which is the source of a bug in some of the YUI widgets we were using which, for reasons we didn’t previously understand, were getting extremely small text. This is now sorted. :-)