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="Burnicki.jpg|37298" />
  </query-continue>
  <query>
    <pages>
      <page pageid="59" ns="0" title="Stanisław R. Brzostkiewicz" />
      <page pageid="78" ns="0" title="Bohdan Kiełczewski" />
      <page pageid="150" ns="0" title="Bernard Krygier" />
      <page pageid="37173" ns="0" title="28 lipca 1851, Całkowite zaćmienie Słońca" />
      <page pageid="37174" ns="0" title="28 lipca 1851, całkowite zaćmienie Słońca" />
      <page pageid="37270" ns="0" title="1986" />
    </pages>
  </query>
</api>