Common yet flexible font size

Body{ Font-face:  Whatever; Font-size: 100%; } Usually defaults to the user's default font size of 16 px.  Then you adjust it up or down with ems.  1em = 100%, .9em = 90%. IE doesn't allow for adjustable font sizes if you style with pixels.  So you have to go this route. Remember to deal with nested font sizes.  .9em of  font that is already set at .9em will be Not be .9em of the original font, but .9em of the .9em size. This is probably another one that is in "CSS the Definitive Guide", but I'm experimenting with CSS techniques *now* and it will take at least one day to go out and get a book. So, to jump ahead, what would be a good way to define a common font face and size for all browsers without getting in the way of users abilities to adjust size if desired. I usually just let the cards fly and just accept the default of each browser, but in my experiments today I noticed this can very dramatically between browsers.  So I was thinking it would be cool to layout a base line that would be consistent across all the browsers, similar to what I do with margins and paddings of the HTML and Body elements.  But I sense it is probably somewhat trickier for lettering. TIA Ian