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="Игры_для_PlayStation_Vita" />
  </query-continue>
  <query>
    <allcategories>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Counter-Strike</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Freeware</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Steam-игры</c>
      <c size="2" pages="2" files="0" subcats="0" xml:space="preserve">Игры, использующие язык Lua</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Игры Valve Software</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Игры для Android</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Игры для Linux</c>
      <c size="0" pages="0" files="0" subcats="0" xml:space="preserve">Игры для Mac OS X</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Игры для PlayStation 3</c>
      <c size="1" pages="1" files="0" subcats="0" xml:space="preserve">Игры для PlayStation Network</c>
    </allcategories>
  </query>
</api>