SOURCE

<div class="box">
  <div class="scale">
    <div class="item">
      16:9
    </div>
  </div>
</div>

.box {
  width: 90%;
  margin: 0 auto;
}

.scale {
  width: 100%;
  padding-bottom: 56.25%;
  height: 0;
  position: relative;
}

.item {
  width: 100%;
  height: 100%;
  background-color: aquamarine;
  position: absolute;
  background: url(https://cdn.jsdelivr.net/gh/lhlyu/pb@master/b/41.jpg);
  background-size: cover;
}