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="20th_Century" />
  </query-continue>
  <query>
    <allcategories>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">11th Century</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">12th Century</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">13th Century</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">14th Century</c>
      <c size="7" pages="7" files="0" subcats="0" xml:space="preserve">15th Century</c>
      <c size="10" pages="10" files="0" subcats="0" xml:space="preserve">16th Century</c>
      <c size="25" pages="25" files="0" subcats="0" xml:space="preserve">17th Century</c>
      <c size="130" pages="130" files="0" subcats="0" xml:space="preserve">18th Century</c>
      <c size="223" pages="223" files="0" subcats="0" xml:space="preserve">19th Century</c>
      <c size="8" pages="8" files="0" subcats="0" xml:space="preserve">1st to 10th Centuries Ce</c>
    </allcategories>
  </query>
</api>