title: Ensure an element retains a specified aspect ratio
body: <p>Is there anyway to specify an aspect ratio in CSS such as 4:3 or 16:9 and then have the div (container) block fit the current window width and height?</p>
  
  <p><img src="https://i.stack.imgur.com/7fP0Y.png" alt="enter image description here"></p>
  
  <p>Quite a simple question hopefully it has a relatively simple answer.</p>
  
  <p><strong><em>Edit:</strong> those still interested a great example of achieving this using only styles would be Bootstrap's <a href="https://v4-alpha.getbootstrap.com/utilities/responsive-helpers/" rel="nofollow noreferrer">responsive helpers</a> classes.</em></p>