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>
  <expandtemplates xml:space="preserve">&lt;blockquote style=&quot;background: white; border: 1px dotted black; padding: 1em;&quot;&gt;''Welcome to the '''SuperTux Wiki Sandbox!''' Please feel free to conduct your experiments with formatting and and editing skills on this page. You may place anything here; however, nothing copyrighted, offensive, or related to spam. To edit this page, click on the '''edit''' button on the top of the page. Nothing is permanent, as this page is cleaned every once in a while.'' &lt;/blockquote&gt;

Mediawiki Update Test

So, no more error message when editing? --[[User:WolfgangB|WolfgangB]] ([[User talk:WolfgangB|talk]]) 19:15, 18 August 2014 (UTC)</expandtemplates>
</api>