title: Firefox: calc() invalid property value
body: <p>I have an element with a line-height set with <code>calc()</code>:</p>
  
  <pre><code>line-height: calc(3rem / 2);
  </code></pre>
  
  <p>Demo: <a href="http://codepen.io/Ghodmode/pen/vLxZZd" rel="nofollow noreferrer">http://codepen.io/Ghodmode/pen/vLxZZd</a></p>
  
  <p>It works fine in Chrome, but Firefox's Developer Tools says that it's an invalid property value.</p>
  
  <p>I'm sure I'm missing something that should be obvious.</p>
  
  <p>Just for reference:</p>
  
  <ul>
  <li><a href="https://developer.mozilla.org/en/docs/Web/CSS/calc" rel="nofollow noreferrer">https://developer.mozilla.org/en/docs/Web/CSS/calc</a></li>
  <li><a href="http://caniuse.com/#search=calc" rel="nofollow noreferrer">http://caniuse.com/#search=calc</a></li>
  </ul>