The situation is a bit murky now because the <center> tag is depracated in HTML 4.01, but will continue to work for sometime. However, it is not supported in XHTML 1.0 Strict DTD which is going to be more widely used as more and more mobile devices are used and you are encouraged not to use the <center> now. What you should do is to use inline CSS (Cascading Style Sheet). You should now use
instead, and you will no longer have to worry about what happens in the future.