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="CH1_TRI2_1.png|7" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:CH1 Map1.png" />
      <f ns="6" title="File:CH1 Map2.png" />
      <f ns="6" title="File:CH1 Map3.png" />
      <f ns="6" title="File:CH1 Map4.png" />
      <f ns="6" title="File:CH1 Map5.png" />
      <f ns="6" title="File:CH1 Map6.png" />
      <f ns="6" title="File:CH1 Preview.png" />
      <f ns="6" title="File:CH1 TRI1 1.png" />
      <f ns="6" title="File:CH1 TRI1 2.png" />
      <f ns="6" title="File:CH1 TRI1 3.png" />
    </allfileusages>
  </query>
</api>