aspect-ratio-center.css

/* per http://stackoverflow.com/questions/20590239/maintain-aspect-ratio-of-div-but-fill-screen-width-and-height-in-css */
div {
  width: 100vw; 
  height: 56.25vw; /* height:width ratio = 9/16 = .5625  */
  background: pink;
  max-height: 100vh;
  max-width: 177.78vh; /* 16/9 = 1.778 */
  margin: auto;
  position: absolute;
  top:0;bottom:0; /* vertical center */
  left:0;right:0; /* horizontal center */
}