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="27184" timestamp="2013-02-05T16:38:19Z" comment="" xml:space="preserve">&lt;h2&gt;Mission statement&lt;/h2&gt;&lt;ul&gt;&lt;li&gt;To promote Systems Thinking and Systems Engineering in the Greater London community&lt;/li&gt;&lt;li&gt;To provide a meeting point between users and providers os Systems Engineering in the Locality&lt;/li&gt;&lt;li&gt;To promote the discussion of local problems where SE might be applied successfully&lt;/li&gt;&lt;li&gt;To promote dialogue across application domains&lt;/li&gt;&lt;li&gt;To promote the identity of the Systems Engineer&lt;/li&gt;&lt;/ul&gt;
See how to make a [[new page]] on the mediawiki help system (http://www.mediawiki.org/wiki/Help:Links)</rev>
        </revisions>
      </page>
    </pages>
  </query>
</api>