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="全民國防" />
  </query-continue>
  <query>
    <allcategories>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">2015論壇</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">2020總統及立委候選人承諾書</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">AI</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">Etherpad</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">Node.js</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">OpenStreetMap</c>
      <c size="10" pages="10" files="0" subcats="0" xml:space="preserve">上課或演講</c>
      <c size="10" pages="10" files="0" subcats="0" xml:space="preserve">作業系統</c>
      <c size="4" pages="3" files="0" subcats="1" xml:space="preserve">偏鄉教育</c>
      <c size="2" pages="1" files="0" subcats="1" xml:space="preserve">內容資源</c>
    </allcategories>
  </query>
</api>