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>
    <pages>
      <page pageid="406" ns="0" title="Exos classiques et autres démonstrations" />
      <page pageid="967" ns="0" title="Leçons d&#039;informatique" />
      <page pageid="185" ns="0" title="Problème de séparation par automate" />
      <page pageid="99" ns="0" title="Théorème de Hahn-Banach" />
    </pages>
  </query>
</api>