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>
    <threads>
      <thread id="1" subject="Discussions about enhancements of the Wiki" modified="20161116142919" />
      <thread id="2" subject="Discontinuing this page." modified="20130907082325" />
      <thread id="3" subject="Move the text from the Category:OpenFOAM guide?" modified="20130825160705" />
      <thread id="4" subject="Discontinuing this page." modified="20130907082325" />
    </threads>
  </query>
</api>