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>
    <allfileusages afcontinue="S_MBoard_v11.jpg|1408" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Carte mere v1.2 tiny.jpg" />
      <f ns="6" title="File:Cycl5 tiny.jpg" />
      <f ns="6" title="File:CycloneIIIv24 sm.jpg" />
      <f ns="6" title="File:CycloneIIIv24 sm.jpg" />
      <f ns="6" title="File:Ex1 logout.jpg" />
      <f ns="6" title="File:Ex1 scriptin.jpg" />
      <f ns="6" title="File:Inputpackets.jpg" />
      <f ns="6" title="File:Logfile.jpg" />
      <f ns="6" title="File:Outputpackets.jpg" />
      <f ns="6" title="File:Photos evaristeIII.jpg" />
    </allfileusages>
  </query>
</api>