It doesn't display properly in the XML Editor because the XML Editor isn't a full browser set. It's close, but you don't see all the effects you'd see in the web output, like drop-down text or togglers, or appropriate font sizing when using relative fonts.
I use ems, as well. I have the body tag set to 1.0em and the <p> tag set to .8ems, which is the size I have for most body text. Headings are 1.6em, 1.4em, etc., depending on what other attributes I set on the heading tag (background color, bottom border, etc.).
When I first set up the project, I get the main styles font sizes set and frequently build the output to see how it looks, testing in both Firefox and Internet Explorer, because (of course) they don't match on everything. And any time I add a new class or modify a tag I hadn't used yet, I test.
One thing to keep in mind with using relative fonts (not a problem with fixed font sizes), but you're really a "victim" of the cascading part of the cascading stylesheet. If you use relative font sizes, then you're going to have to set up several complex selectors, such as
td p and
ul li p and others. Using my example, if my <td> and <p> tags are both set to .8em, but then I put a <p> tag inside a <td> tag, then the size of the <p> tag is relative to the size of the <td> tag, so .8em * .8em = .64em for the size of the <p> tag. Since I don't want a <p> tag inside a <td> tag to be smaller than a regular paragraph, I have a complex selector set up (td p) that specifies that the font-size of a <p> tag inside a <td> tag is 1.0em. It's not really 1.0em like the <body> tag, it's actually saying that it's the same size as the parent tag.
