title: create responsive div for banner
body: <p>how to create responsive div like img-responsive class in bootstrap but for a div <br>
  that when window size change, height of div element change related to its width..</p>
  
  <p>like this website slider "<a href="http://sabanet.ir" rel="nofollow">http://sabanet.ir</a>"
  when you change window size the slider keep the ratio of height to width...</p>
  
  <p>am i should use javascript to do it or i can do it with css or bootstrap?</p>
  
  <pre><code>    &lt;div class="responsive-banner"&gt;
          &lt;!-- height/width === k --&gt;
      &lt;/div&gt;
  </code></pre>