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>
    <allpages apcontinue="Codex_De_Iztapalapa" />
  </query-continue>
  <query>
    <allpages>
      <p pageid="116" ns="0" title="Calendar Correlation" />
      <p pageid="63" ns="0" title="Calendario Mexicano No. 7 de Veytia" />
      <p pageid="62" ns="0" title="Calendario Mexicano de Veytia No. 5" />
      <p pageid="72" ns="0" title="Catecismo Gomez de Orozco" />
      <p pageid="19" ns="0" title="Cempoalilhuitl" />
      <p pageid="78" ns="0" title="Codex Azoyu Number 1" />
      <p pageid="73" ns="0" title="Codex Azoyu Number 2" />
      <p pageid="69" ns="0" title="Codex Boturini" />
      <p pageid="74" ns="0" title="Codex Chavero" />
      <p pageid="56" ns="0" title="Codex Cholula" />
    </allpages>
  </query>
</api>