SOURCE

<div id="box">
  <div id="test-padding">padding</div>
  <div id="test-margin">margin</div>
  <div id="test-after">margin</div>
</div>

#box {
  width: 500px;
  height: 200px;
}
#test-padding {
  width: 50%;
  height: 0;
  padding-bottom: 50%;
  background: #c00;
}
#test-margin {
  width: 50%;
  margin-top: 50%;
  background: #c00;
  border: 1px solid #000;
}

#test-after {
  width: 50%;
  overflow: hidden;
  background: #c00;
}
#test-after:after {
  content: '';
  display: block;
  margin-top: 100%;
}