Sorry if I was vague in my example description; anyway, it didn't apply specifically to my layout.
Here's a better example: Say I have an image, whatever it may be, and I want the top-left corner to be 50 px to the left of the center of the document and 20 px from the top of the document, positioned absolutely. What would the code for that be?
Anyway, Gil's version of the page works nicely (thanks a lot), so I'll probably go with that for now. However, I still would like to know how to offset from the center with CSS for future reference.
P.S. Gil, what do the <tbody></tbody> tags do? They seem to have some significance, but I've never heard of them.