title: how to get clientWidth of firstChild
body: <p>I want to get client width of firstChild</p>
  
  <p><strong>Css</strong></p>
  
  <pre><code>.ot{width:100px;height:200px}
  </code></pre>
  
  <p><strong>JS</strong></p>
  
  <pre><code>var ra = document.getElementsByClassName("r")[0];
  var n = ra.firstChild.clientHeight;
  var ca = n ;
  </code></pre>
  
  <p><strong>HTML</strong></p>
  
  <pre><code>&lt;div class="r"&gt;&lt;div class="ot"&gt;&lt;/div&gt;&lt;/div&gt;
  </code></pre>
  
  <p>want to get the height of <code>ot</code> through class <code>r</code>, why not <code>ca</code> is showing 200  </p>