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="コーカサス神話" />
  </query-continue>
  <query>
    <allcategories>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Mesopotamian mythology</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Sumerian epic heroes</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">イルカ</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">エトルリア神話</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">エラム</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">オセチア神話</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">オリーブ</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">ギリシア神話</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">クルアーン</c>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">コロン</c>
    </allcategories>
  </query>
</api>