title: Parse errors when using calc with rem and px
body: <p>I have the following CSS:</p>
  
  <pre class="lang-css prettyprint-override"><code>body {
    width: calc(700px + 2 * 4rem);
  }
  
  .someclass {
    margin: calc(2rem - 2px) 0;
  }
  </code></pre>
  
  <p>When I run this through the W3C CSS validation, I get </p>
  
  <blockquote>
    <p>Value Error : width Parse Error + 2 * 4rem)<br>
    Value Error : margin Parse Error - 2px) 0</p>
  </blockquote>
  
  <p>What’s causing this? All the units I’m using in <code>calc()</code> are length units, and <code>calc()</code> should be able to handle it.</p>