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="云璟月:菠萝派计划" />
  </query-continue>
  <query>
    <allcategories>
      <c size="3" pages="3" files="0" subcats="0" xml:space="preserve">Android开发</c>
      <c size="4" pages="4" files="0" subcats="0" xml:space="preserve">BootstrapMediaWiki设置</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Windows技巧</c>
      <c size="2" pages="1" files="0" subcats="1" xml:space="preserve">不可索引页面</c>
      <c size="2" pages="0" files="0" subcats="2" xml:space="preserve">中国古代</c>
      <c size="1" pages="0" files="0" subcats="1" xml:space="preserve">中国古代文学</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">二分与分治</c>
      <c size="8" pages="0" files="0" subcats="8" xml:space="preserve">云璟月</c>
      <c size="37" pages="37" files="0" subcats="0" xml:space="preserve">云璟月:圣诞树计划</c>
      <c size="15" pages="15" files="0" subcats="0" xml:space="preserve">云璟月:模板</c>
    </allcategories>
  </query>
</api>