title: How to both center a <div> and have it use fixed positioning?
body: <p>I have a div with some text on my page, and I want it to be at the bottom. I did this using <strong>fixed positioning</strong>: </p>
  
  <pre><code>div#popup{
      position: fixed;
      bottom: 0;
  </code></pre>
  
  <p>But I also want it to be centered. I tried giving it a width of 40% and auto margins, but that doesn't work (it doesn't work with the combination of the above code) : </p>
  
  <pre><code> div#popup{
      position: fixed;
      bottom: 0;
      width: 40%;
      margin-left: auto;
      margin- right: auto;
  </code></pre>
  
  <p>How can I achieve this? </p>
  
  <p>Thanks.</p>