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="Chess_Qd44.png" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Certificato1.png" />
      <f ns="6" title="File:Certificato2.png" />
      <f ns="6" title="File:Chess Bd44.png" />
      <f ns="6" title="File:Chess Bl44.png" />
      <f ns="6" title="File:Chess Kd44.png" />
      <f ns="6" title="File:Chess Kl44.png" />
      <f ns="6" title="File:Chess Nd44.png" />
      <f ns="6" title="File:Chess Nl44.png" />
      <f ns="6" title="File:Chess Pd44.png" />
      <f ns="6" title="File:Chess Pl44.png" />
    </allfileusages>
  </query>
</api>