title: center div in screen without fixing width and height
body: <p>I use bootstrap to have a responsive design and I want to center one div in the screen</p>
  
  <p>I tested that : </p>
  
  <pre><code>style="position:fixed;top:50%;left:50%;"
  </code></pre>
  
  <p>but the top left corner of the div is in the center, I don't want put negative values of margin to make him reponsive</p>
  
  <p>how an I solve this issue</p>