You can use XSLT in a web application in at least three basic ways:

1. Use XSLT offline to style the content (e.g. via Ant).

2. Create an XSLT stylesheet for all your content and run that "on the server".

3. Create XSLT stylesheets for specific documents and run that when those documents are requested.

Good implementations of server technology will allow you to cache styled content.