title: Center a position:fixed element
body: <p>I would like to make a <code>position: fixed;</code> popup box centered to the screen with a dynamic width and height. I used <code>margin: 5% auto;</code> for this. Without <code>position: fixed;</code> it centers fine horizontally, but not vertically. After adding <code>position: fixed;</code>, it's even not centering horizontally.</p>
  
  <p>Here's the complete set:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="false" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.jqbox_innerhtml {
      position: fixed;
      width: 500px;
      height: 200px;
      margin: 5% auto;
      padding: 10px;
      border: 5px solid #ccc;
      background-color: #fff;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="jqbox_innerhtml"&gt;
      This should be inside a horizontally
      and vertically centered box.
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>How do I center this box in screen with CSS?</p>