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>
    <allcategories>
      <c size="6" pages="6" files="0" subcats="0" xml:space="preserve">CH391L S12</c>
      <c size="8" pages="8" files="0" subcats="0" xml:space="preserve">CH391L S13</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">CH391L S14</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">CH391L S14 Index of all CH391L/S14 pages</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">NSC 110H S15 Microbes as Machines</c>
      <c size="15" pages="15" files="0" subcats="0" xml:space="preserve">Pages with broken file links</c>
    </allcategories>
  </query>
</api>