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="Base_conversion_table.png" />
  </query-continue>
  <query>
    <allfileusages>
      <f ns="6" title="File:Base16 symbols.png" />
      <f ns="6" title="File:Base4 add table.png" />
      <f ns="6" title="File:Base4 horiz tables.png" />
      <f ns="6" title="File:Base4 mult table.png" />
      <f ns="6" title="File:Base4 operations.png" />
      <f ns="6" title="File:Base4 symbols.png" />
      <f ns="6" title="File:Base4 symbols 2.png" />
      <f ns="6" title="File:Base4 tables.png" />
      <f ns="6" title="File:Base4 to others.png" />
      <f ns="6" title="File:Base4 vert tables.png" />
    </allfileusages>
  </query>
</api>