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In the following DocBook 4 document, the content of the <listitem> element does not match the rules of the DocBook 4 schema (docbookx.dtd).

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
     "http://www.docbook.org/xml/4.4/docbookx.dtd">
<article>
    <title>Article Title</title>
    <sect1>
      <title>Section1 Title</title>
      <itemizedlist>
        <listitem>
          <link>a link here</link>
        </listitem>
      </itemizedlist>
    </sect1>
</article>

The Validate Document action will return the following error:

Unexpected element "link". The content of the parent element type must match 
"(calloutlist|glosslist|bibliolist|itemizedlist|orderedlist|segmentedlist|simplelist
|variablelist|caution|important|note|tip|warning|literallayout|programlisting
|programlistingco|screen|screenco|screenshot|synopsis|cmdsynopsis|funcsynopsis
|classsynopsis|fieldsynopsis|constructorsynopsis|destructorsynopsis|methodsynopsis
|formalpara|para|simpara|address|blockquote|graphic|graphicco|mediaobject|mediaobjectco
|informalequation|informalexample|informalfigure|informaltable|equation|example|figure
|table|msgset|procedure|sidebar|qandaset|task|anchor|bridgehead|remark|highlights
|abstract|authorblurb|epigraph|indexterm|beginpage)+".

This error message is a little more difficult to understand, so understanding of the syntax or processing rules for the DocBook XML DTD <listitem> element is recommended. However, the error message does offer a clue as to the source of the problem, indicating that The content of element type must match.

Fortunately, most standards-based DTDs, XML Schemas, and Relax NG schemas are supplied with reference documentation. This enables you to read about the element. In this case, you should learn about the child elements of <listitem> and their nesting rules. Once you have correctly inserted the required child element and nested it in accordance with the XML rules, the document will become valid.