<!DOCTYPE html>
<style>
html { 
  height: 100%;
}

body {
  height: 100%;
  margin: 0px;
}

.container { 
  height: 100%;
  display: flex;
}

.middle { 
  flex-grow: 1;
  flex-direction: column;
}

.inner { 
  background-color: green;
  height: -webkit-fill-available;
  width: 100px;
}
</style>
<!-- PASS if the box does expand all the way to the bottom of viewport -->
<div class=container><div class=middle><div class=inner>
