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="Pantallazo-Opciones.png" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Datos0.png" />
      <f ns="6" title="File:Doro-Fear no evil.jpg" />
      <f ns="6" title="File:Ed2k-ff3.png" />
      <f ns="6" title="File:Ed2k-op.png" />
      <f ns="6" title="File:Firefox ed2k link.PNG" />
      <f ns="6" title="File:Jmule screen5.png" />
      <f ns="6" title="File:Limits aMuleSVN Fedora Gnome.png" />
      <f ns="6" title="File:Opera preferences programs.PNG" />
      <f ns="6" title="File:Opera protocols ed2k.PNG" />
      <f ns="6" title="File:Pantallazo-Cola-de-descarga.png" />
    </allfileusages>
  </query>
</api>