Browser Problems with the XML Prolog
http://www.webstandards.org/learn/articles/prolog_problems/

Web Standards Project
2007

Abstract:

Some browsers have difficulty upon encountering the XML Prolog. In some cases, the browser will render all the markup as text. In other cases, when a browser has some XML support, it might attempt to render the document as an XML tree. To avoid these problems, many practicing web professionals prefer to leave the prolog off. This table will help you make that decision by showing you which browsers have known problems with the XML prolog.