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 pageid="39" ns="0" title="CrucialのMX200とBX100とBX100とM550のスペック比較" />
      <page pageid="72" ns="0" title="Windows 8.1" />
      <page pageid="173" ns="0" title="HGST Ultrastar He10 10TB HDD" />
    </pages>
  </query>
</api>