title: Get the width of an element and use it to set its height
body: <p>I'm using Bootstrap's grid system, and I set the <code>&lt;iframe&gt;</code> width to 90%. </p>
  
  <p>The problem is how can I set the height of <code>&lt;iframe&gt;</code> accrodingly? 
  For example, I want a 16:9 size. But I dont know how can I get the value of the width</p>
  
  <p>Code may look like:</p>
  
  <pre><code>iframe
   width: 90%
   height: $(width)*9/16
  </code></pre>