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>
  <compare fromrevid="1" torevid="2" xml:space="preserve">&lt;tr&gt;&lt;td colspan=&quot;2&quot; class=&quot;diff-lineno&quot;&gt;Line 1:&lt;/td&gt;
&lt;td colspan=&quot;2&quot; class=&quot;diff-lineno&quot;&gt;Line 1:&lt;/td&gt;&lt;/tr&gt;
&lt;tr&gt;&lt;td class='diff-marker'&gt;−&lt;/td&gt;&lt;td class='diff-deletedline'&gt;&lt;div&gt;'''MediaWiki has been successfully installed.'''&lt;/div&gt;&lt;/td&gt;&lt;td class='diff-marker'&gt;+&lt;/td&gt;&lt;td class='diff-addedline'&gt;&lt;div&gt;'''MediaWiki has been successfully installed&lt;ins class=&quot;diffchange diffchange-inline&quot;&gt;, but DokuWiki FTW!&lt;/ins&gt;.'''&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr&gt;&lt;td class='diff-marker'&gt;&amp;#160;&lt;/td&gt;&lt;td class='diff-context'&gt;&lt;/td&gt;&lt;td class='diff-marker'&gt;&amp;#160;&lt;/td&gt;&lt;td class='diff-context'&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr&gt;&lt;td class='diff-marker'&gt;&amp;#160;&lt;/td&gt;&lt;td class='diff-context'&gt;&lt;div&gt;Consult the [http://meta.wikimedia.org/wiki/Help:Contents User's Guide] for information on using the wiki software.&lt;/div&gt;&lt;/td&gt;&lt;td class='diff-marker'&gt;&amp;#160;&lt;/td&gt;&lt;td class='diff-context'&gt;&lt;div&gt;Consult the [http://meta.wikimedia.org/wiki/Help:Contents User's Guide] for information on using the wiki software.&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;
</compare>
</api>