The DTD for XHTML 1.1 is split up in several files (modules). All of these can be downloaded from below. Note that some of them has been modified by me to make them work locally (no references back to W3C). For your convenience I've included them all in one zip-archive.
Source
Main DTD
<!DOCTYPE SYSTEM "xhtml11.dtd">)
Modules
- http://www.w3.org/TR/xhtml-modularization/DTD/*.mod
Ruby Module
- http://www.w3.org/TR/ruby/*.mod
Character Entity Sets (included in "XHTML Character Entities Module")
- http://www.w3.org/TR/xhtml-modularization/DTD/*.ent
SGML related files
Example file (this file)
Download file (zip-archive containing all the files from above)