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>
    <pages>
      <page ns="0" title="API" missing="" />
      <page pageid="1" ns="0" title="Main Page">
        <revisions>
          <rev user="Trubo-Warrior" timestamp="2019-08-08T13:14:41Z" comment="" contentformat="text/x-wiki" contentmodel="wikitext" xml:space="preserve">If you visit this site, you are probably looking for the math- or physics-wiki. Follow those links: 

[http://dmath.ferienserie.ch/index.php?title=Main_Page Math-Wiki]

[http://dphys.ferienserie.ch/index.php?title=Main_Page Physics-Wiki]

We protected them with a password for various reasons.</rev>
        </revisions>
      </page>
    </pages>
  </query>
</api>