DTD

DTDs for Apertium XML formats are available in SVN within these folders:

• https://svn.code.sf.net/p/apertium/svn/trunk/apertium/apertium/
• https://svn.code.sf.net/p/apertium/svn/trunk/lttoolbox/lttoolbox/

You can search inside these folders for the DTD files (*.dtd). There are also .rng files that are automatically converted from the .dtd's, but the dtd's are the primary sources.