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="case-sensitive" canonical="Media" xml:space="preserve">媒体文件</ns>
      <ns id="-1" case="first-letter" canonical="Special" xml:space="preserve">特殊</ns>
      <ns id="0" case="case-sensitive" subpages="" content="" nonincludable="" xml:space="preserve" />
      <ns id="1" case="case-sensitive" canonical="Talk" nonincludable="" xml:space="preserve">讨论</ns>
      <ns id="2" case="case-sensitive" subpages="" canonical="User" nonincludable="" xml:space="preserve">用户</ns>
      <ns id="3" case="case-sensitive" canonical="User talk" nonincludable="" xml:space="preserve">用户讨论</ns>
      <ns id="4" case="case-sensitive" subpages="" canonical="Project" nonincludable="" xml:space="preserve">Blueidea</ns>
      <ns id="5" case="case-sensitive" canonical="Project talk" nonincludable="" xml:space="preserve">Blueidea讨论</ns>
      <ns id="6" case="case-sensitive" canonical="File" nonincludable="" xml:space="preserve">文件</ns>
      <ns id="7" case="case-sensitive" canonical="File talk" nonincludable="" xml:space="preserve">文件讨论</ns>
      <ns id="8" case="first-letter" subpages="" canonical="MediaWiki" nonincludable="" xml:space="preserve">MediaWiki</ns>
      <ns id="9" case="first-letter" canonical="MediaWiki talk" nonincludable="" xml:space="preserve">MediaWiki讨论</ns>
      <ns id="10" case="case-sensitive" subpages="" canonical="Template" xml:space="preserve">模板</ns>
      <ns id="11" case="case-sensitive" canonical="Template talk" nonincludable="" xml:space="preserve">模板讨论</ns>
      <ns id="12" case="case-sensitive" subpages="" canonical="Help" nonincludable="" xml:space="preserve">帮助</ns>
      <ns id="13" case="case-sensitive" canonical="Help talk" nonincludable="" xml:space="preserve">帮助讨论</ns>
      <ns id="14" case="case-sensitive" subpages="" canonical="Category" nonincludable="" xml:space="preserve">分类</ns>
      <ns id="15" case="case-sensitive" canonical="Category talk" nonincludable="" xml:space="preserve">分类讨论</ns>
      <ns id="100" case="case-sensitive" canonical="URL" nonincludable="" xml:space="preserve">URL</ns>
      <ns id="101" case="case-sensitive" canonical="URL talk" nonincludable="" xml:space="preserve">URL talk</ns>
    </namespaces>
  </query>
</api>