I don't think it is semantics, setting vertical margin: auto here has just no effect.

It's worth noting that the last example with margin:auto can be used for centering within any div that has position: relative (there's nothing special about the whole page). It also works for intrinsic dimensions, so you can center an image without knowing its size, see http://jsfiddle.net/jdudek/hfbnS/1/.

There are a few more useful techniques not covered by this article, for example:

* line-height + vertical-align: middle + display: inline-block (which allows you to center vertically an element with dynamic height)

* display: table-cell (mentioned by others in this thread, although without stating its disadvantages).

While I appreciate the effort, calling the article "complete guide to centering a div" is an overstatement.