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>
    <namespaces>
      <ns id="-2" case="first-letter" canonical="Media" xml:space="preserve">Media</ns>
      <ns id="-1" case="first-letter" canonical="Special" xml:space="preserve">Especial</ns>
      <ns id="0" case="first-letter" subpages="" content="" xml:space="preserve" />
      <ns id="1" case="first-letter" subpages="" canonical="Talk" xml:space="preserve">Conversa</ns>
      <ns id="2" case="first-letter" subpages="" canonical="User" xml:space="preserve">Usuario</ns>
      <ns id="3" case="first-letter" subpages="" canonical="User talk" xml:space="preserve">Conversa usuario</ns>
      <ns id="4" case="first-letter" subpages="" canonical="Project" xml:space="preserve">Guifi.net - Wiki Galego</ns>
      <ns id="5" case="first-letter" subpages="" canonical="Project talk" xml:space="preserve">Conversa Guifi.net - Wiki Galego</ns>
      <ns id="6" case="first-letter" canonical="File" xml:space="preserve">Ficheiro</ns>
      <ns id="7" case="first-letter" subpages="" canonical="File talk" xml:space="preserve">Conversa ficheiro</ns>
      <ns id="8" case="first-letter" subpages="" canonical="MediaWiki" xml:space="preserve">MediaWiki</ns>
      <ns id="9" case="first-letter" subpages="" canonical="MediaWiki talk" xml:space="preserve">Conversa MediaWiki</ns>
      <ns id="10" case="first-letter" canonical="Template" xml:space="preserve">Modelo</ns>
      <ns id="11" case="first-letter" subpages="" canonical="Template talk" xml:space="preserve">Conversa modelo</ns>
      <ns id="12" case="first-letter" subpages="" canonical="Help" xml:space="preserve">Axuda</ns>
      <ns id="13" case="first-letter" subpages="" canonical="Help talk" xml:space="preserve">Conversa axuda</ns>
      <ns id="14" case="first-letter" canonical="Category" xml:space="preserve">Categoría</ns>
      <ns id="15" case="first-letter" subpages="" canonical="Category talk" xml:space="preserve">Conversa categoría</ns>
      <ns id="90" case="first-letter" canonical="Thread" xml:space="preserve">Tópico</ns>
      <ns id="91" case="first-letter" canonical="Thread talk" xml:space="preserve">Tópico discussão</ns>
      <ns id="92" case="first-letter" canonical="Summary" xml:space="preserve">Resumo</ns>
      <ns id="93" case="first-letter" canonical="Summary talk" xml:space="preserve">Resumo discussão</ns>
    </namespaces>
  </query>
</api>