title: CSS value as mathematical expression
body: <p>Is there anyway that I can write a css value as mathematical expression?
  Example:</p>
  
  <pre><code>div{
      height: 50% + 30px;
      width: 40em - 5px;
     }
  </code></pre>
  
  <p>If there is, it would be perfect. <br/>
  <strong>PS:</strong> I don't want to do it with <em>JS</em> or <em>JQuery</em>.</p>