Show the strcture of aspect ratio.

<!-- 16:9 aspect ratio -->
 <div class="embed-responsive embed-responsive-16by9">   
<iframe class="embed-responsive-item" src="..."></iframe>
 </div><!-- 4:3 aspect ratio --> 
<div class="embed-responsive embed-responsive-4by3"> 
<iframe class="embed-responsive-item" src="..."></iframe>
 </div>