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="Coord_system_CA_0.svg.png|3316" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Backgammon lg.png" />
      <f ns="6" title="File:Bad-Job-Brewery 2015-11-30-1101-1.jpg" />
      <f ns="6" title="File:Bicycle Alexandra Motz.jpg" />
      <f ns="6" title="File:Bluehost WP auto-update.jpg" />
      <f ns="6" title="File:Boxarea.png" />
      <f ns="6" title="File:CSS-Background.png" />
      <f ns="6" title="File:Calvin-seibert 12529579735 099058d5ac z.jpg" />
      <f ns="6" title="File:Calvin-seibert 12529579735 099058d5ac z.jpg" />
      <f ns="6" title="File:Calvin-siebert 12529579735 099058d5ac z.jpg" />
      <f ns="6" title="File:Cerveza-Muscular.jpg" />
    </allfileusages>
  </query>
</api>