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>
    <allcategories accontinue="Pages_with_broken_file_links" />
  </query-continue>
  <query>
    <allcategories>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Billing Experts</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Category name</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">General Medical Terms</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Health standards</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">International Classification of Diseases</c>
      <c size="5" pages="5" files="0" subcats="0" xml:space="preserve">Medical Associations</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Medical Billing</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Medical Billing,Medical Coding</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">Medical Billing Companies</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">Medical Coding</c>
    </allcategories>
  </query>
</api>