XML Web GUI comes with a variety of DTDs, but of course you can use any other DTD for your documents or even write an own.
For a wide choice of XML Applications and DTDs the open directory project has an own category.
If you would like to write your own DTD, the ZVON DTD Tutorial is a recommended ressource.
You have three way to adress a DTD
External HTTP reference. You can adress a DTD by referring to the URL of a DTD on another server.
Example 3.5. adressing the Docbook DTD
The advantage is that you can be sure that the DTD is always up-to-date. The disadvantage is that every access to XML Web GUI will cause network traffic.
File reference. You can also adress a DTD by referring to a file in your filesystem.
Example 3.6. adressing the WML DTD
The advantage of this is that it is very fast and requires no HTTP transfer. The disadvantage is that the created XML files are not distributable unless on the client's computer there is the DTD at the same path.
Internal HTTP reference. The recommended adressing of DTD is via HTTP, but to your own server. (Or the server XML Web GUI is running on).
Example 3.7. adressing the Simplesite DTD
If you repace 127.0.0.1 with the ip-adress or domain-name of your server, the the DTD will be available for validating as long as your server is running and XML Web GUI will not need a network conncection to validate the files.