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 gafcontinue="Base_conversion_table.png" />
  </query-continue>
  <query>
    <pages>
      <page pageid="18" ns="6" title="File:Base16 symbols.png" />
      <page pageid="14" ns="6" title="File:Base4 add table.png" />
      <page pageid="30" ns="6" title="File:Base4 horiz tables.png" />
      <page pageid="15" ns="6" title="File:Base4 mult table.png" />
      <page pageid="26" ns="6" title="File:Base4 operations.png" />
      <page pageid="16" ns="6" title="File:Base4 symbols.png" />
      <page pageid="17" ns="6" title="File:Base4 symbols 2.png" />
      <page pageid="28" ns="6" title="File:Base4 tables.png" />
      <page pageid="27" ns="6" title="File:Base4 to others.png" />
      <page pageid="29" ns="6" title="File:Base4 vert tables.png" />
    </pages>
  </query>
</api>