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="B|623" />
  </query-continue>
  <query>
    <pages>
      <page pageid="538" ns="106" title="MP:Qu&#039;est-ce qu&#039;un œuvre ?" />
      <page pageid="561" ns="106" title="MP:Notre expérience du Je à l&#039;œuvre" />
      <page pageid="603" ns="106" title="MP:Au dos de nos images. Journal d&#039;un frère Dardenne" />
      <page pageid="607" ns="106" title="MP:L&#039;œuvre selon MarieK" />
      <page pageid="608" ns="106" title="MP:L&#039;œuvre selon Suricate" />
      <page pageid="611" ns="106" title="MP:L&#039;œuvre selon Caroline" />
      <page pageid="612" ns="106" title="MP:L&#039;œuvre selon Patricia" />
      <page pageid="613" ns="106" title="MP:L&#039;œuvre selon Dany" />
      <page pageid="618" ns="106" title="MP:Niki de Saint Phalle, une œuvre en JEu" />
      <page pageid="621" ns="106" title="MP:Notes et carnets de Fabienne Verdier" />
    </pages>
  </query>
</api>