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="Img00.png|1001" />
  </query-continue>
  <query>
    <pages>
      <page pageid="2" ns="0" title="Página principal" />
      <page pageid="973" ns="0" title="Cizalla" />
      <page pageid="974" ns="0" title="Plegadora" />
      <page pageid="975" ns="0" title="EDAR" />
      <page pageid="982" ns="0" title="Residuos metálicos" />
      <page pageid="990" ns="0" title="Regla 20-20-20" />
      <page pageid="991" ns="0" title="Ruido" />
      <page pageid="996" ns="0" title="Efectos del trabajo" />
      <page pageid="999" ns="0" title="Equipo de Protección Individual" />
      <page pageid="1000" ns="0" title="Protección colectiva" />
    </pages>
  </query>
</api>