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="API" missing="" />
      <page pageid="1" ns="0" title="Main Page">
        <revisions>
          <rev user="MediaWiki default" anon="" timestamp="2014-09-10T12:26:09Z" comment="" contentformat="text/x-wiki" contentmodel="wikitext" xml:space="preserve">&lt;strong&gt;MediaWiki has been successfully installed.&lt;/strong&gt;

Consult the [//meta.wikimedia.org/wiki/Help:Contents User's Guide] for information on using the wiki software.

== Getting started ==
* [//www.mediawiki.org/wiki/Manual:Configuration_settings Configuration settings list]
* [//www.mediawiki.org/wiki/Manual:FAQ MediaWiki FAQ]
* [https://lists.wikimedia.org/mailman/listinfo/mediawiki-announce MediaWiki release mailing list]
* [//www.mediawiki.org/wiki/Localisation#Translation_resources Localise MediaWiki for your language]</rev>
        </revisions>
      </page>
    </pages>
  </query>
</api>