title: Fit, scale and center image responsively to browser window (CSS)
body: <p>How do I have an image, irrespective of portrait or landscape:</p>
  
  <ul>
  <li>centered responsively both horizontally and vertically  (remains centered as browser window is resized)</li>
  <li>occupies a fixed fraction of the browser window, scaling responsively</li>
  <li>preserves aspect ratio as browser window is resized</li>
  </ul>