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="Check_mark_23x20_04.svg" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Blue question mark (italic).svg" />
      <f ns="6" title="File:Bouncing Snowball.png" />
      <f ns="6" title="File:Buggered graph.png" />
      <f ns="6" title="File:Bulbgraph.svg" />
      <f ns="6" title="File:Cacti favicon.ico" />
      <f ns="6" title="File:Cacti favicon demo.png" />
      <f ns="6" title="File:Cancelled cross.svg" />
      <f ns="6" title="File:Cancelled process mini.svg" />
      <f ns="6" title="File:Check mark.svg" />
      <f ns="6" title="File:Check mark 23x20 02.svg" />
    </allfileusages>
  </query>
</api>