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="Butter.jpg|1005" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:B.B. King in 2009.jpg" />
      <f ns="6" title="File:Bach-Bild von &#039;Gebel&#039;, vor 1798.jpg" />
      <f ns="6" title="File:Bach-Bild von &#039;Gebel&#039;, vor 1798.jpg" />
      <f ns="6" title="File:Big Ben Clock London UK.jpg" />
      <f ns="6" title="File:Bigger, better, stronger.jpg" />
      <f ns="6" title="File:Bill to withdraw jurisdiction re draft.png" />
      <f ns="6" title="File:Bloch-SermonOnTheMount.jpg" />
      <f ns="6" title="File:Bobby Fischer smiling 1972.jpg" />
      <f ns="6" title="File:Bobby Fischer smiling 1972.jpg" />
      <f ns="6" title="File:Branches of Christianity.jpg" />
    </allfileusages>
  </query>
</api>