You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query-continue>
    <alltransclusions gatcontinue="Ch:Bibliography|1317" />
  </query-continue>
  <query>
    <pages>
      <page pageid="1285" ns="0" title="Introduction" />
      <page pageid="1304" ns="0" title="Information for developers" />
      <page pageid="1311" ns="0" title="Preface" />
      <page pageid="1318" ns="0" title="Lecture: Introduction" />
      <page pageid="1325" ns="0" title="Lecture Index" />
      <page pageid="1347" ns="0" title="Lecture: System Modeling" />
      <page pageid="1432" ns="0" title="Appendices" />
      <page pageid="1668" ns="0" title="Supplement: Biomolecular Feedback Systems" />
      <page pageid="2392" ns="0" title="BFS:Archived news" />
    </pages>
  </query>
</api>