Someone who wants to represent everything in xml, and do everything with XML. This type of person can be spotted when you notice that the "<" and ">" keys on his/her keyboard are worn out.

''Proponent != fanatic''

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WhyCyTagIsntMyFavouriteProgrammingLanguage explains how some innocent developers are being tortured by an xml fanatic.
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See also: JavaProponent, AllaireColdFusion