If I have the following Docbook 5 XML:
<article xmlns="http://docbook.org/ns/docbook" xmlns:xl="http://www.w3.org/1999/xlink" version="5.0">
<title>Test</title>
<section>
<title>LS command</title>
<para xml:id="ls">
This command is a synonym for <command linkend="dir">DIR</command> command.
</para>
<para xml:id="dir">
This command is a synonym for <command linkend="ls">LS</command> command.
</para>
<para>
<application xl:href="http://www.gnu.org/software/emacs/">Emacs</application>
</para>
</section>
</article>
and I attempt to validate it against the Docbook 5 DTD downloaded from here with:
xmllint --noout --dtdvalid docbook.dtd test.xml
I get the following error:
test.xml:1: element article: validity error : No declaration for attribute xmlns:xl of element article
test.xml:11: element application: validity error : No declaration for attribute href of element application
However, if I change the xl namespace to xlink, like so:
<article xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0">
<title>Test</title>
<section>
<title>LS command</title>
<para xml:id="ls">
This command is a synonym for <command linkend="dir">DIR</command> command.
</para>
<para xml:id="dir">
This command is a synonym for <command linkend="ls">LS</command> command.
</para>
<para>
<application xlink:href="http://www.gnu.org/software/emacs/">Emacs</application>
</para>
</section>
</article>
Everything validates just fine. I got the xl namespace from the Docbook documentation here (and the example of using an article with an application that had a xref is straight from that documentation).
So why does xl fail when xlink succeeds?
