I'm guessing that you are using Saxon for processing.
This issue was discussed a couple of weeks ago:
https://lists.oasis-open.org/archives/docbook-apps/201304/msg00025.html
Bob Stayton
Sagehill Enterprises
From: Lars Vogel
Sent: Wednesday, April 17, 2013 1:52 PM
To: DocBook Apps
Subject: [docbook-apps] epub generation result has incorrect
DocType
Hello,
I'm generating epub from Docbook input and getting during the validation
one error and one warning with regards to the Doctype.
[java] Epubcheck Version 3.0
[java] Validating against EPUB version 2.0
[java] ERROR:
/home/vogella/workspace/docu/de.vogella.publishing/../output/epub/book.epub/OEBPS/content.opf:
Obsolete or irregular DOCTYPE statement. The DOCTYPE can be removed.
[java] WARNING:
/home/vogella/workspace/docu/de.vogella.publishing/../output/epub/book.epub/OEBPS/toc.ncx:
Irregular DOCTYPE: found '-//W3C//DTD XHTML 1.1//EN', expecting '<!DOCTYPE
ncx PUBLIC "-//NISO//DTD ncx 2005-1//EN" "http://www.daisy.org/z3986/2005/ncx-2005-1.dtd">'.
My input looks simular to the XML file listed below.
Any advice how to remove this error? I'm using DocBook XSL 1.77 for the
transformation.
Best regards, Lars
<?xml version="1.0"?>
<book>
<bookinfo>
<title>Java programming with
the Eclipse IDE</title>
<author>
<firstname>Lars</firstname>
<surname>Vogel</surname>
</author>
<copyright>
<year>2007</year>
<holder> Lars
Vogel</holder>
</copyright>
<pubdate>17.02.2013</pubdate>
<releaseinfo>Third
edition
</releaseinfo>
<mediaobject
role="cover">
<imageobject
id="cover-image">
<imagedata
fileref="images/cover.png" align="center" />
</imageobject>
</mediaobject>
</bookinfo>
<part>
<title>Eclipse
Overview</title>
<chapter>
<title>Eclipse overview
and terminology
</title>
<para> This chapter gives a
short introduction into
the Eclipse
project and the Eclipse IDE.
</para>
</chapter>
<part>
</book>
