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"?>
    <allfileusages gafcontinue="Castellum_Fectio2.jpg" />
      <page pageid="545" ns="6" title="Bestand:Bacchus door leonardo davinci.jpg" />
      <page pageid="238" ns="6" title="Bestand:Balustrade valkhof.jpg" />
      <page pageid="525" ns="6" title="Bestand:Bataven.jpg" />
      <page pageid="437" ns="6" title="Bestand:Boerinn logo.png" />
      <page pageid="301" ns="6" title="Bestand:Boot-buiten.jpg" />
      <page pageid="445" ns="6" title="Bestand:Camelonschoen.jpg" />
      <page pageid="215" ns="6" title="Bestand:Casino romeinen-0426.jpg" />
      <page pageid="216" ns="6" title="Bestand:Casino romeinen-0430.jpg" />
      <page pageid="210" ns="6" title="Bestand:Casino romeinen-0447.jpg" />
      <page pageid="570" ns="6" title="Bestand:Castellum Fectio1.jpg" />