macports-ports/textproc/docbook-xml/files

Please note that the docbook-xml port is simply a meta-port
that installs all versions of the docbook-xml Document Type
Definitions (DTDs) that MacPorts ships.

When MacPorts ships new versions of the docbook-xml DTDs, this
port will be updated so that the new DTD is installed on your
system when you upgrade outdated ports.