responsive 16:9 ratio with plain css

style.css

// 16:9 aspect ratio (9/16 = 56.25%), use 75% for 4:3
// you can also add padding-top to add some extra space

.image-wrapper {
    background-color: #000;
    width: 100%;
    padding-bottom: 56.25%;
    position: relative;
}
.image-wrapper img {
    position: absolute;
    width: 100%;
    top: 0;
    left: 0;
    height: 100%;
}