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="iso|4" />
  </query-continue>
  <query>
    <pages>
      <page pageid="4" ns="0" title="C言語解説" />
      <page pageid="11" ns="0" title="quick exitによるプログラムの終了処理" />
      <page pageid="12" ns="0" title="gcc" />
      <page pageid="20" ns="0" title="配列の初期化 指示付きの初期化指定子" />
      <page pageid="21" ns="0" title="配列の要素数を調べる" />
      <page pageid="22" ns="0" title="配列の初期化" />
      <page pageid="34" ns="0" title="clang" />
      <page pageid="44" ns="0" title="プログラムの入出力" />
    </pages>
  </query>
</api>