title: set a ul size according to the div that containes it
body: <p>I need to have some elements to fill only the space of the element that contains it, for exemple, this is the code:</p>
  
  <pre><code>&lt;header class="w_head"&gt;
          &lt;div class="container-fluid"&gt;
  
              &lt;ul class="w_social text-right"&gt;
                  &lt;li&gt;&lt;a href="#"&gt;&lt;img src="images/Twitter_logo_blue.png"/&gt;&lt;/a&gt;&lt;/li&gt;
                  &lt;li&gt;&lt;a href="#"&gt;&lt;img src="images/g+images-2013-01-15/g+ icon/Desktop/44dp/2x/btn_white.png"/&gt;&lt;/a&gt;&lt;/li&gt;
              &lt;/ul&gt;
  
          &lt;/div&gt;
      &lt;/header&gt;
  </code></pre>
  
  <p>and the styling:</p>
  
  <pre><code>html, body {
      margin: 0;
      padding: 0;
      height: 100%;
      width: 100%;
  }
  .container-fluid{
      position:relative;
  }
  
  .w_head{
      background-color: #66757f;
      height: 2.6em;
      width: 100%;
      position: relative;
  }
  </code></pre>
  
  <p>I need the ul to automatically be placed only inside and according to the header size and not the whole document.</p>