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="Guadrian_Pegasus.jpg" />
  </query-continue>
  <query>
    <pages>
      <page pageid="463" ns="6" title="Soubor:Barbarian.jpg" />
      <page pageid="426" ns="6" title="Soubor:Bishop of Black Battle.jpg" />
      <page pageid="442" ns="6" title="Soubor:C2.jpg" />
      <page pageid="410" ns="6" title="Soubor:Castle Pinehurst.jpg" />
      <page pageid="418" ns="6" title="Soubor:Conan.jpg" />
      <page pageid="419" ns="6" title="Soubor:Congratulations !.jpg" />
      <page pageid="421" ns="6" title="Soubor:Coraks spirit.jpg" />
      <page pageid="414" ns="6" title="Soubor:Cuisinart.jpg" />
      <page pageid="416" ns="6" title="Soubor:Devil Kings osobne.jpg" />
      <page pageid="460" ns="6" title="Soubor:Draci snem.jpg" />
    </pages>
  </query>
</api>