<style>
.fill { 
  width: 10px;
  background-color: green;
  height: -webkit-fill-available;
  position: absolute;
}
.relative-containing-block {
  position: relative;
}
</style>
<div class=fill></div>
<div class=relative-containing-block>
  <div class=fill style="background-color: red"></div>
</div>