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>
    <allfileusages gafcontinue="Coord_system_CA_0.svg.png|3316" />
  </query-continue>
  <query>
    <pages>
      <page pageid="1509" ns="0" title="Perfect information (game theory)" />
      <page pageid="1764" ns="0" title="Cerveza Muscular" />
      <page pageid="2213" ns="0" title="Bad Job Brewery" />
      <page pageid="2617" ns="0" title="Box model (CSS)" />
      <page pageid="2877" ns="0" title="Background (CSS)" />
      <page pageid="3355" ns="0" title="Calvin Seibert" />
      <page pageid="3358" ns="0" title="Sand art" />
      <page pageid="3629" ns="0" title="Alexandra Motz" />
      <page pageid="3992" ns="0" title="WordPress Dashboard Updates" />
    </pages>
  </query>
</api>