I thank you for the comments and complements. I am still working on the site, experimenting with different things. I added a custom DTD, mostly as a way to keep track of the elements I am adding and removing from the different variants of the XML files, and I will see how that goes.

My new experiment is trying to make the site cross-platform. Unfortunately, there seem to be no cell phones that support XML+XSLT, so the transformation has to be done server side. This is a discovery I wish I had made before I developed an XSL template that could handle desktop and cell phone formats. I think the next step is to create a PHP script that can get the XML file and transform it according to the XSL template and return the resulting page to the client. Any advice on a method to do so using PHP? For instance, is that something I should be able to accomplish with an existing PHP library, or am I going to have to custom build a solution?

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The best way to learn anything, is to question everything.