Это улучшает принятый ответ:
.box {
margin-top: 1em;
margin-bottom: 1em;
background-color: #CCC;
}
.fixed-ar::before {
content: "";
float: left;
width: 1px;
margin-left: -1px;
}
.fixed-ar::after {
content: "";
display: table;
clear: both;
}
.fixed-ar-16-9::before {
padding-top: 56.25%;
}
.fixed-ar-3-2::before {
padding-top: 66.66%;
}
.fixed-ar-4-3::before {
padding-top: 75%;
}
.fixed-ar-1-1::before {
padding-top: 100%;
}
.width-50 {
display: inline-block;
width: 50%;
}
.width-20 {
display: inline-block;
width: 20%;
}
<div class="box fixed-ar fixed-ar-16-9">16:9 full width</div>
<hr>
<div class="box fixed-ar fixed-ar-16-9 width-50">16:9</div>
<hr>
<div class="box fixed-ar fixed-ar-16-9 width-20">16:9</div>
<div class="box fixed-ar fixed-ar-3-2 width-20">3:2</div>
<div class="box fixed-ar fixed-ar-4-3 width-20">4:3</div>
<div class="box fixed-ar fixed-ar-1-1 width-20">1:1</div>
<hr>
<div class="box fixed-ar fixed-ar-16-9 width-20">16:9</div>
<div class="box fixed-ar fixed-ar-16-9 width-50">16:9</div>