This site is designed well. I don't mean it looks great (although I personally think it's nice)--that's subjective. But objectively, it is designed well, because it does three very important things:
These things are good, because they make the page useable by everybody. The standards are registered and (surprise, surprise) standard, meaning they're held by a body outside the control of any software company with an agenda. Any modern standards-compliant web browser should render the site about the same, and I don't have to do any stupid tricks for different browsers.
The design also fails gracefully, meaning that if you look at this site in Netscape 3 or Lynx, even though they're primitive and ancient, it's still quite usable. Look at it in a good browser, like anything Mozilla-based, Opera, or anything KHTML-based, and it'll look really nice.
It'll look somewhat less nice in most versions of Internet Explorer, because Microsoft's support of even years-old standards sucks. Ask them why, but don't use their browser to encourage them when there are lots of better free alternatives.
I'm also not trying to force you to use any particular font size or window width; you can easily adjust either, and the page looks fine. In fact, feel free to override my stylesheet if you want--the markup is simple, so it'll work fine.
If everybody designed their sites this way (and the good ones do), the web would be a nicer place, a place where sites were a whole lot more likely to look the same in any browser, and a lot friendlier for non-Microsoft browsers.
By the way, there's no particular reason to use XHTML 1.1 -- XHTML 1.0 Transitional or even HTML 4.01 are just fine. I'm just using the fancy stuff for personal practice, and because I think it's fun.