title: get total width of children elements
body: <p>I have some HTML tags:</p>
  
  <pre><code>&lt;div id="container"&gt;
     &lt;input type="text" id="a"&gt;
     &lt;textarea id="b"&gt;&lt;/textarea&gt;
     &lt;div id="c" style="width:200px"&gt;&lt;/div&gt;
     &lt;div id="d" style="width:20%"&gt;&lt;/div&gt;
  &lt;/div&gt; 
  </code></pre>
  
  <p><code>#a</code> and <code>#b</code> have no css width attribute. How could I calculate the total width of <code>container</code>'s children elements in pixel?</p>