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="Click_bar_state_action.png|40" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Character.png" />
      <f ns="6" title="File:Character set.png" />
      <f ns="6" title="File:Character set - key maps.png" />
      <f ns="6" title="File:Character set - key maps.png" />
      <f ns="6" title="File:Character set - key maps.png" />
      <f ns="6" title="File:Character set - slot.png" />
      <f ns="6" title="File:Character set - slot.png" />
      <f ns="6" title="File:Character set - slot.png" />
      <f ns="6" title="File:Click Bar Buttons page.png" />
      <f ns="6" title="File:Click bar button action.png" />
    </allfileusages>
  </query>
</api>