Here are some things you can do with XSLT stylesheets in XPD.

Post-Processors

Once a stylesheet has finished rendering, you can pass off the output of that stylesheet to another stylesheet by adding a post-process instruction at the top of the document, e.g.:

<?post-process href="forms.xslt" type="text/xsl" ?>

Form Handlers

Before XPD begins to render a document, it will call all of the form handlers for the document. A form handler is a .NET class that implements the XPD.FormHandler interface. To register a form handler with an XSLT stylesheet, include an attribute starting with pre-process- whose value is the name of the class implementing FormHandler. e.g.:

<xsl:stylesheet ... 
	pre-process-checkloggedin="MyNamespace.MyFormHandler, MyAssembly"
	

The form handler will look like:

namespace MyNamespace {
	public class MyFormHandler : XPD.FormHandler {
		public bool Process(XPD.Context context) {
			// Check System.Web.HttpContext.Current.Request...
			// Return true to abort all further processing
			// of this document.
			return false;
		}
	}
}
	

And it should be in the assembly MyAssembly.dll in the bin directory.