title: Centering an absolute positioned div
body: <p>I need to center this div that is wrapped inside another div:</p>
  
  <p>Div to be centered:</p>
  
  <pre><code>.post-username {
      padding: 10px;
      background: #000;
      border-radius: 4px;
      position: absolute;
      top: 0;
  }
  </code></pre>
  
  <p>It should be centered inside of this:</p>
  
  <pre><code>.post {
      background: #3e4758;
      overflow: hidden;
      border-radius: 4px;
      width: 270px;
      height: 330px;
      position: relative;
  }
  </code></pre>