In the context of Milind's original question, I would just like to emphasis that XML is not an altenative for CSS. XML is just a data representation format and you will still need a mechanism( XSL/XSLT ) to present that data in a readable format to your clients.
Also think about this - if you don't need to
exchange your data with third-parties and/or process the data received from an external source, you may not need to use XML at all. XML might be an overkill in a closed system.
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Ajith Kallambella M. Sun Certified Programmer for the Java�2 Platform.
IBM Certified Developer - XML and Related Technologies, V1.