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;Linia 1:&lt;/td&gt;
&lt;td colspan=&quot;2&quot; class=&quot;diff-lineno&quot;&gt;Linia 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;okładka numeru &lt;del class=&quot;diffchange diffchange-inline&quot;&gt;10&lt;/del&gt;/2006&amp;lt;br /&amp;gt;Copyright by &amp;quot;&lt;del class=&quot;diffchange diffchange-inline&quot;&gt;Mówią Wieki&lt;/del&gt;&amp;quot;, 2006&lt;/div&gt;&lt;/td&gt;&lt;td class='diff-marker'&gt;+&lt;/td&gt;&lt;td class='diff-addedline'&gt;&lt;div&gt;okładka numeru &lt;ins class=&quot;diffchange diffchange-inline&quot;&gt;9&lt;/ins&gt;/2006&amp;lt;br /&amp;gt;Copyright by &amp;quot;&lt;ins class=&quot;diffchange diffchange-inline&quot;&gt;W drodze&lt;/ins&gt;&amp;quot;, 2006&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;
</compare>
</api>