Container with fixed aspect ratio. Size is based on the current viewport.

aspect-ratio.scss

$ratio-height:  3;
$ratio-width:   4;
$margin:        0.1;

div#selector {
    width:      (1 - $margin) * 100vw;
    height:     (1 - $margin) * $ratio-height / $ratio-width * 100vw;
    max-height: (1 - $margin) * 100vh;
    max-width:  (1 - $margin) * $ratio-width / $ratio-height * 100vh;
}