Given a sticky footer such as that on Ryan Fait\'s site with a fixed pixel height, is it possible to center, both horizontally and vertically, variable-size content in the space
I would suggest looking at Bobby van der Sluis's article on Footers at A List Apart.
Example #7 at the end of his article shows a vertically centered block. It does rely on scripting, but it is truly minimal.
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You can also use a single-cell table to accomplish vertical centering. Incorporating it with Ryan Fait's sticky footer would give you something like this:
Vertically Centered!
This block will remain centered. Just needs that one table cell wrapping.