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-continue>
    <alltransclusions gatcontinue="class|241" />
  </query-continue>
  <query>
    <pages>
      <page pageid="5" ns="0" title="C++ライブラリ" />
      <page pageid="8" ns="0" title="Boost" />
      <page pageid="155" ns="0" title="抽象構文木" />
      <page pageid="173" ns="0" title="const cast" />
      <page pageid="174" ns="0" title="static cast" />
      <page pageid="175" ns="0" title="reinterpret cast" />
      <page pageid="176" ns="0" title="dynamic cast" />
      <page pageid="177" ns="0" title="NULL" />
      <page pageid="187" ns="0" title="std::move" />
      <page pageid="204" ns="0" title="g++をUbuntuへインストールする" />
    </pages>
  </query>
</api>