Not necessarily...
Almost every modern browsers support native xml+xsl transformation.

I've wrote, use, and released a set of 2 classes, that allow an easy output of xml, to select what I call a "rendering set" (imagine a version for ipod/pda and another "rich media") and to determine if the client should receive a server-transformed html file, or if xml+xsl can be sent directly.

You can see it in action on Quixml::Quick Xml and Xsl for PHP.
Try to look at the pages source from firefox or safari....

And an example is in the class documentation for the ClXml class => Docs For Class clXml (you loose the navigation by going directly to this page.)