title: why isn't margin:auto working?
body: <p>Just trying to centre an image which i have placed using HTML. Simply trying to use Margin:auto but it it isn't working?</p>
  
  <p>Here is the code:</p>
  
  <pre><code>#indexheader {
      margin: 0 auto;
      margin-top:5%;
      width:190%;
      height: 190%;
  }
  </code></pre>