title: Should I define CSS margins in pixels or ems? Why? When?
body: <p>We have a CSS file with some rules similar to the following:</p>
  
  <pre><code>.directory-result ul
  {
      margin-top: 20px;
      width: 60em;
  
  }
  
  .about-text
  {
      margin-top: 1em;
      margin-bottom: 1em;
  }
  </code></pre>
  
  <p>Everything is working ok, but we're wondering specifically about the inconsistencies between the margin-top values. One is 20px and the other is 1em.</p>
  
  <p>Which is the best one to go with? What are the points I should consider when deciding which to use? Thanks.</p>