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>
    <pages>
      <page ns="0" title="Op:スレイヤーズVSオーフェン" missing="" />
      <page ns="0" title="Op:魔術士オーフェンはぐれ旅" missing="" />
      <page ns="0" title="Op:魔術士オーフェンまわり道" missing="" />
      <page ns="0" title="Op:魔術士オーフェン・無謀編" missing="" />
      <page pageid="14" ns="0" title="イラストレーターの一覧" />
    </pages>
  </query>
</api>