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="Pantallazo-Opciones.png" />
  </query-continue>
  <query>
    <pages>
      <page pageid="4420" ns="6" title="File:Datos0.png" />
      <page pageid="5015" ns="6" title="File:Doro-Fear no evil.jpg" />
      <page pageid="4417" ns="6" title="File:Ed2k-ff3.png" />
      <page pageid="4418" ns="6" title="File:Ed2k-op.png" />
      <page pageid="4477" ns="6" title="File:Firefox ed2k link.PNG" />
      <page pageid="4937" ns="6" title="File:Jmule screen5.png" />
      <page pageid="4404" ns="6" title="File:Limits aMuleSVN Fedora Gnome.png" />
      <page pageid="4478" ns="6" title="File:Opera preferences programs.PNG" />
      <page pageid="4479" ns="6" title="File:Opera protocols ed2k.PNG" />
      <page pageid="4400" ns="6" title="File:Pantallazo-Cola-de-descarga.png" />
    </pages>
  </query>
</api>