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-continue>
    <alltransclusions gatcontinue="Bausteindesign2" />
  </query-continue>
  <query>
    <pages>
      <page pageid="779" ns="10" title="Vorlage:Balken Pixel" />
      <page pageid="780" ns="10" title="Vorlage:Balken Prozent" />
      <page pageid="781" ns="10" title="Vorlage:Balken Prozent 2" />
      <page pageid="782" ns="10" title="Vorlage:Balken schmal Pixel" />
      <page pageid="727" ns="10" title="Vorlage:Balkendiagramm" />
      <page pageid="783" ns="10" title="Vorlage:Balkendiagramm/Doku" />
      <page pageid="784" ns="10" title="Vorlage:Balkendiagramm/Meta" />
      <page pageid="638" ns="10" title="Vorlage:Baustein Material" />
      <page pageid="690" ns="10" title="Vorlage:Bausteindesign" />
      <page pageid="785" ns="10" title="Vorlage:Bausteindesign11" />
    </pages>
  </query>
</api>