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>
    <allpages apcontinue="Class_Diagram" />
  </query-continue>
  <query>
    <allpages>
      <p pageid="353" ns="0" title="Behavioural Analysis" />
      <p pageid="374" ns="0" title="Behavioural Design" />
      <p pageid="271" ns="0" title="Block Diagram" />
      <p pageid="350" ns="0" title="Block Interface Design" />
      <p pageid="346" ns="0" title="Blockdiagram" />
      <p pageid="221" ns="0" title="Class Analysis" />
      <p pageid="302" ns="0" title="Class Analysis - real" />
      <p pageid="262" ns="0" title="Class Analysis - realisation" />
      <p pageid="222" ns="0" title="Class Candidates" />
      <p pageid="269" ns="0" title="Class Design" />
    </allpages>
  </query>
</api>