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>
    <allimages gaicontinue="550px-Apollonian-network.svg.png" />
  </query-continue>
  <query>
    <pages>
      <page pageid="3421" ns="6" title="File:1024px-Flammarion.jpg" />
      <page pageid="3418" ns="6" title="File:1024px-Vehicle simulator.jpg" />
      <page pageid="3428" ns="6" title="File:1280px-2005-06-25 Tiles together.jpg" />
      <page pageid="3399" ns="6" title="File:14-236-LunarGrailMission-OceanusProcellarum-Rifts-Overall-20141001.jpg" />
      <page pageid="3488" ns="6" title="File:1600 Himmelsscheibe von Nebra sky disk anagoria.jpg" />
      <page pageid="3256" ns="6" title="File:200px-Gnomon.svg.png" />
      <page pageid="3150" ns="6" title="File:320px-Pascal&#039;s triangle 5.svg.png" />
      <page pageid="3257" ns="6" title="File:3DGraphicsGnomon.png" />
      <page pageid="3407" ns="6" title="File:414px-Paolo uccello, studio di vaso in prospettiva 02.jpg" />
      <page pageid="3496" ns="6" title="File:498px-Venn diagram gr la ru.svg.png" />
    </pages>
  </query>
</api>