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="DMS_OH_O2.png" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:BNU LAI 2010.png" />
      <f ns="6" title="File:Boxes removed from EMEP mask.png" />
      <f ns="6" title="File:C2H6 0-8km TAO&amp;GC 2006.pdf" />
      <f ns="6" title="File:C2H6 OH.png" />
      <f ns="6" title="File:CONV H2O2.png" />
      <f ns="6" title="File:Cherry pick 1.jpg" />
      <f ns="6" title="File:Cherry pick 2.jpg" />
      <f ns="6" title="File:Cherry pick 3.jpg" />
      <f ns="6" title="File:DMS NO3.png" />
      <f ns="6" title="File:DMS OH.png" />
    </allfileusages>
  </query>
</api>