Lots of designers have used this trick to vertically center blocks of known size in browser windows but, as Jon Hicks points out, some content may be cropped if the browser window is too small. Though it requires a little extra markup, my solution addresses this problem by positioning a shiv shim instead of the content itself.

While working on an earlier version of xa, I discovered that IE4 incorrectly parses commented asterisks (/***/) in stylesheets. Check out the example to see how to exploit it. I've since submitted the hack (which does validate) to the relevant websites.