_ratio-mixin.scss

// The mixin
@mixin set-ratio($width, $height, $selectors...) {

    // needed on the parent so the absolute childs reference it
    position: relative;

    // create our ratio with the psuedo before
    &:before {
        display: block;
        content: "";
        width: 100%;
        padding: 0 0 percentage($height/$width) 0;
        height: 0;
    }

    // check length of selectors and provide default if non exist
    @if length($selectors) == 0 {
        $selectors: img
    }

    // loop through selectors and create direct descendent selector
    @each $selector in $selectors {
      > #{$selector} {
            z-index: 100;
            position: absolute;
            top: 0;
            right: 0;
            bottom: 0;
            left: 0;
            width: 100%;
            height: 100%;
        }
    }
}

// You might have a structure like so

// <div class="parent">
//     <div class="container">
//         <div class="child"></div>
//     </div>
// </div>

// Using Mixin

.parent {
    // first 2 arguments is the ratio you want to use
    // Last argument or (arguments) are the elements you want to apply the required css above to.
    @include set-ratio(16, 9, div);
}