The EPUB format does not support references from an EPUB file to
another EPUB file. Therefore any references to “external” Texinfo
manuals should resolve to an external URL. texi2any
produces these links with HTML cross-reference configuration
(see HTML Cross-reference Configuration: htmlxref.cnf). Since the links in the
resulting EPUB are incorrect if no information is found for the
cross-references, texi2any issues a warning by default for
missing cross-references information. If these warnings are unwanted,
set CHECK_HTMLXREF to 0.