Alright, I\'ve search the jQuery docs (needs somebody devoted to maintaining), I\'ve searched SO, and I\'ve searched Google. I can\'t find the answer to this question.
This is not a complete answer to your question but it may be a working solution to caclulate the em values. I adapted this function from here. And here is the updated fiddle.
$.fn.emWidth = function(){
var wpx = this.width();
var temp = $('').css({
width:'1em',
position: 'absolute'
});
this.append(temp);
var oneEm = temp.width();
temp.remove();
var value = wpx/oneEm;
return Math.round(value*100)/100;
};