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 atcontinue="Scenario_Type" />
  </query-continue>
  <query>
    <alltransclusions>
      <t ns="10" title="Template:Documentation" />
      <t ns="10" title="Template:Font color" />
      <t ns="10" title="Template:Scenario Task" />
      <t ns="10" title="Template:Scenario Task Parents" />
      <t ns="10" title="Template:Scenario Task Translation" />
      <t ns="10" title="Template:Scenario Tool" />
      <t ns="10" title="Template:Scenario Tool Translation" />
      <t ns="10" title="Template:Scenario Tool Translation Action" />
      <t ns="10" title="Template:Scenario Translation Action" />
      <t ns="10" title="Template:Scenario Translations" />
    </alltransclusions>
  </query>
</api>