As Gordaen said, you need to specify the left and right margin to be 'auto' - this is mentioned in the article, BTW.
The problem is a design flaw with Internet Explorer.
Elements are rendered using rectangles, and these rectangles have margins, and naturally the way to center a rectangle inside another is to give it left and right margins of equal width. This works given that the element is block-level and not inline, and not positioned.
However Internet Explorer does a lot of things a different way, and one of them is to use "text-align" property to center elements instead of just text. So the article first told you the proper way to do it, then told you the way to do it for Internet Explorer.
Internet Explorer gets it right though if you trigger its standards-compliance mode.
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