title: CSS Div not centering
body: <p>My intention is to center a header <code>&lt;div&gt;</code>, but i'm not able to do it.</p>
  
  <pre><code>header {
     height: 54px;
     margin:0px auto;
     width: 1150px;
     background: #13171B;
     position: fixed;
     left: 0;
     top: 0;
     z-index: 990;
     margin-bottom:10px;
  }
  </code></pre>
  
  <p>Please help me.</p>