The XML page: http://cvs.apache.org/viewcvs.cgi/cocoon-2.1/src/documentation/xdocs/faq/book.xml
If you use the rendering view: http://cvs.apache.org/viewcvs.cgi/*checkout*/cocoon-2.1/src/documentation/xdocs/faq/book.xml?rev=1.1
or the text/plain view (and here behaves IE wrongly): http://cvs.apache.org/viewcvs.cgi/*checkout*/cocoon-2.1/src/documentation/xdocs/faq/book.xml?rev=1.1&content-type=text/plain
IE resolves the link "../dtd/book-cocoon-v10.dtd" pointing to http://cvs.apache.org/viewcvs.cgi/*checkout*/cocoon-2.1/src/documentation/xdocs/dtd/book-cocoon-v10.dtd
(which is automatically redirected to http://cvs.apache.org/viewcvs.cgi/cocoon-2.1/src/documentation/xdocs/dtd/Attic/book-cocoon-v10.dtd?hidecvsroot=0&search=None&hideattic=1&sortby=file&logsort=date&diff_format=u)
And there you don't find the DTD, but the Attic CVS description of the DTD, which is a HTML page, which has a reference to another DTD.
In conclusion: It's an issue we simply can not fix besides by removing all links except 'view'. But I guess we won't do this as there is still the possibility to view the page with IE. 2 of 3 links should at least work, 1 does work.
Joerg