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="Balcony_House.jpg|4474" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:BINNEY 016.w.jpg" />
      <f ns="6" title="File:BL11 w.jpg" />
      <f ns="6" title="File:BL12 w.jpg" />
      <f ns="6" title="File:BL13 w.jpg" />
      <f ns="6" title="File:BU06-001 H673-5-5736 1w.jpg" />
      <f ns="6" title="File:BVH NewYearCard 1990 1w.jpg" />
      <f ns="6" title="File:BVH NewYearCard 1991 1w.jpg" />
      <f ns="6" title="File:BX01-004 7507-39-28 11w.jpg" />
      <f ns="6" title="File:BX01-220 9510-14-34 1w.jpg" />
      <f ns="6" title="File:BX01-232 UnknownOrigin c2006 02 1w.jpg" />
    </allfileusages>
  </query>
</api>