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>
    <allpages gapcontinue="Template_measures" />
  </query-continue>
  <query>
    <pages>
      <page pageid="499" ns="0" title="Tajo. Improvement of ecological state of the Tajo and tributaries riverside affected by the spill of kaolin, at Poveda de la Sierra and Taravilla (Guadalajara)" contentmodel="wikitext" pagelanguage="en" touched="2014-05-05T06:14:53Z" lastrevid="3841" counter="20347" length="11828" />
      <page pageid="6685" ns="0" title="Take the catchment perspective" contentmodel="wikitext" pagelanguage="en" touched="2015-10-15T19:07:36Z" lastrevid="29668" counter="2932" length="522" new="" />
      <page pageid="160" ns="0" title="Template casestudies" contentmodel="wikitext" pagelanguage="en" touched="2010-06-20T11:47:53Z" lastrevid="4021" counter="19583" length="1971" />
      <page pageid="746" ns="0" title="Template factsheet" contentmodel="wikitext" pagelanguage="en" touched="2010-07-26T09:09:08Z" lastrevid="4070" counter="20898" length="6379" />
    </pages>
  </query>
</api>