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>
  <warnings>
    <expandtemplates xml:space="preserve">Because no values have been specified for the prop parameter, a legacy format has been used for the output. This format is deprecated, and in the future, a default value will be set for the prop parameter, causing the newformat to always be used.</expandtemplates>
  </warnings>
  <expandtemplates xml:space="preserve">[[:Wikipedia Mundana:Sandbox]]</expandtemplates>
</api>