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HTML is good for debugging, but probably is not suitable for your application.
See complete documentation, or API help for more information. 
<?xml version="1.0" encoding="utf-8"?>
<api>
  <query>
    <namespaces>
      <ns id="-2">Media</ns>
      <ns id="-1">Special</ns>
      <ns id="0" />
      <ns id="1" subpages="">Talk</ns>
      <ns id="2" subpages="">User</ns>
      <ns id="3" subpages="">User talk</ns>
      <ns id="4">SSRG Annoki</ns>
      <ns id="5" subpages="">SSRG Annoki talk</ns>
      <ns id="6">Image</ns>
      <ns id="7" subpages="">Image talk</ns>
      <ns id="8">MediaWiki</ns>
      <ns id="9" subpages="">MediaWiki talk</ns>
      <ns id="10">Template</ns>
      <ns id="11" subpages="">Template talk</ns>
      <ns id="12">Help</ns>
      <ns id="13" subpages="">Help talk</ns>
      <ns id="14">Category</ns>
      <ns id="15" subpages="">Category talk</ns>
    </namespaces>
  </query>
</api>