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 gafcontinue="Chess_Qd44.png" />
  </query-continue>
  <query>
    <pages>
      <page pageid="162" ns="6" title="File:Certificato1.png" />
      <page pageid="163" ns="6" title="File:Certificato2.png" />
      <page pageid="242" ns="6" title="File:Chess Bd44.png" />
      <page pageid="243" ns="6" title="File:Chess Bl44.png" />
      <page pageid="244" ns="6" title="File:Chess Kd44.png" />
      <page pageid="245" ns="6" title="File:Chess Kl44.png" />
      <page pageid="246" ns="6" title="File:Chess Nd44.png" />
      <page pageid="247" ns="6" title="File:Chess Nl44.png" />
      <page pageid="248" ns="6" title="File:Chess Pd44.png" />
      <page pageid="249" ns="6" title="File:Chess Pl44.png" />
    </pages>
  </query>
</api>