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">Mídia</ns>
      <ns id="-1" case="first-letter" canonical="Special" xml:space="preserve">Especial</ns>
      <ns id="0" case="first-letter" content="" xml:space="preserve" />
      <ns id="1" case="first-letter" subpages="" canonical="Talk" xml:space="preserve">Discussão</ns>
      <ns id="2" case="first-letter" subpages="" canonical="User" xml:space="preserve">Usuário</ns>
      <ns id="3" case="first-letter" subpages="" canonical="User talk" xml:space="preserve">Usuário Discussão</ns>
      <ns id="4" case="first-letter" subpages="" canonical="Project" xml:space="preserve">IDB Secure Gateway</ns>
      <ns id="5" case="first-letter" subpages="" canonical="Project talk" xml:space="preserve">IDB Secure Gateway Discussão</ns>
      <ns id="6" case="first-letter" canonical="File" xml:space="preserve">Arquivo</ns>
      <ns id="7" case="first-letter" subpages="" canonical="File talk" xml:space="preserve">Arquivo Discussão</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">MediaWiki Discussão</ns>
      <ns id="10" case="first-letter" canonical="Template" xml:space="preserve">Predefinição</ns>
      <ns id="11" case="first-letter" subpages="" canonical="Template talk" xml:space="preserve">Predefinição Discussão</ns>
      <ns id="12" case="first-letter" subpages="" canonical="Help" xml:space="preserve">Ajuda</ns>
      <ns id="13" case="first-letter" subpages="" canonical="Help talk" xml:space="preserve">Ajuda Discussão</ns>
      <ns id="14" case="first-letter" canonical="Category" xml:space="preserve">Categoria</ns>
      <ns id="15" case="first-letter" subpages="" canonical="Category talk" xml:space="preserve">Categoria Discussão</ns>
    </namespaces>
  </query>
</api>