title: Center fixed div with dynamic width (CSS)
body: <p>I have a div that will have this CSS:</p>
  
  <pre class="lang-css prettyprint-override"><code>#some_kind_of_popup
  {
      position: fixed;
      top: 100px;
      min-height: 300px;
      width: 90%;
      max-width: 900px;
  }
  </code></pre>
  
  <p>Now, how can i make this div centered? I can use <code>margin-left: -450px; left: 50%;</code> but this will only work when the screen is > 900 pixels. After that (when the window is &lt; 900 pixels), it will no longer be centered. </p>
  
  <p>I can of course do this with some kind of js, but is there a "more correct" of doing this with CSS?  </p>