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 ns="0" title="Routage statique IPv6" missing="" />
      <page pageid="323" ns="0" title="ClamAV" />
      <page pageid="299" ns="0" title="Commandes reseaux Linux" />
      <page pageid="300" ns="0" title="Configurer reseau Windows" />
      <page pageid="333" ns="0" title="Debug Messagerie" />
      <page pageid="291" ns="0" title="Routage IPv6" />
      <page pageid="55" ns="0" title="Serveur DNS" />
      <page pageid="322" ns="0" title="SpamAssassin" />
    </pages>
  </query>
</api>