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="Baikonur_seal.png" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="Файл:B.A.Sagyntaev.jpg" />
      <f ns="6" title="Файл:BE-Russian-Turkestan-map.jpg" />
      <f ns="6" title="Файл:BRDM-2 (1964) owned by James Stewart pic6.JPG" />
      <f ns="6" title="Файл:BRICS heads of state and government hold hands ahead of the 2014 G-20 summit in Brisbane, Australia (Agencia Brasil).jpg" />
      <f ns="6" title="Файл:BRM-1K (1).jpg" />
      <f ns="6" title="Файл:BTA Bank logo.jpg" />
      <f ns="6" title="Файл:Baba 0084.JPG" />
      <f ns="6" title="Файл:Baba 15.jpg" />
      <f ns="6" title="Файл:Baba Yaga.jpg" />
      <f ns="6" title="Файл:Baidibek2.jpg" />
    </allfileusages>
  </query>
</api>