The Forrest dtd is documented on their site:

* [Basic documentation and howto|http://forrest.apache.org/dtdx/dtd-docs.html]. Note that we use 2.0.
* [A visual introduction|http://forrest.apache.org/dtdx/document-v20.html]

Note that you must write ''forrest validate'' before checking in. Running 
the site locally (''f8'') is not enough, since your own computer is more 
tolerant than the Forrest dtd.