Having defined any new modules, you can now define a DTD for your new document type.
<!ENTITY % MyModule
PUBLIC "-//MyOrg/Widgets 1.0//EN"
"http://www.myorg.com/DTD/widgets1_0.dtd">
%MyModule;
<!ENTITY % XHTML11-strict
PUBLIC "-//W3C/XHTML 1.1 Strict"//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11-s.dtd">
%XHTML11-strict;