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="Building_Cave_of_Crystals.png|248" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:BarraDeMenú Botón Héroes.png" />
      <f ns="6" title="File:Barra de Mana.png" />
      <f ns="6" title="File:Barra de menu magia.PNG" />
      <f ns="6" title="File:Barra exp Eroe.png" />
      <f ns="6" title="File:Bazaar Dragon Eggs.png" />
      <f ns="6" title="File:Bazaar items.png" />
      <f ns="6" title="File:Bazaar items.png" />
      <f ns="6" title="File:Building Breeding Pit.png" />
      <f ns="6" title="File:Building Breeding Pit.png" />
      <f ns="6" title="File:Building Cave of Crystals.png" />
    </allfileusages>
  </query>
</api>