Ratio box in css

ratio_box.css

.a {
  width: 900px;
  position: relative;
}
.a:before {
  content: '';
  display: block;
  width: 100%;
  padding-bottom: 56.25%;
  position: relative;
}
.b {
  position: absolute;
  bottom: 0;
  right: 0;
  left: 0;
  top: 0;
  background: #ccc;
}
.content {
  position: absolute;
  top: 50%;
  left: 50%;
  width: 200px;
  text-align: center;
  height: 100px;
  line-height: 100px;
  margin: -50px 0 0 -100px;
  background: #777;
  color: #fff;
  font-size: 20px;
}

ratio_box.html

<div class="a"><div class="b">
  <div class="content">16:9</div>
</div></div>