We all want things to display the same on all browsers. PCs and Macs too!
I sometimes run into problems with picky clients who "see" what I have done differently. It's generally a PC/Mac thing (not always) where fonts display differently.
I check as best as I can on a PC - FF, IE 7-9, Chrome & Safari, but can't check Mac browsers.
Part of the problem is that I like to use (and my clients love) the TinyMCE Advanced plugin that allows you to use Fonts and Font sizes in the WP editor. One of the difficulties is that the TMA uses keyword font sizes which display differently in different browsers. But that's not really a subject for here... I'll take that up with Ozz someday.
I came across a couple of interesting posts, that I'd like Flynn or the moderators to comment on. I'm wondering if this is as "Green Tea" mentions and if it might be something to roll into Atahualpa. You'd be specifying a "pixel-sized" font in the WP editor but it would still be an em and so still have em type properties. (pun intended ;-)
Here are the links I found interesting...
Love to hear opinions!