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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>
    <categories clcontinue="3019|Zionism" />
  </query-continue>
  <query>
    <pages>
      <page pageid="3019" ns="0" title="Albert Einstein">
        <categories>
          <cl ns="14" title="Category:Agnostics" />
          <cl ns="14" title="Category:American Jews" />
          <cl ns="14" title="Category:European Jews" />
          <cl ns="14" title="Category:Featured articles" />
          <cl ns="14" title="Category:German People" />
          <cl ns="14" title="Category:Jewish People" />
          <cl ns="14" title="Category:Nobel Laureates in Physics" />
          <cl ns="14" title="Category:Physicists" />
          <cl ns="14" title="Category:Religion and Politics" />
          <cl ns="14" title="Category:Scientific Pioneers" />
        </categories>
      </page>
    </pages>
  </query>
</api>