<!DOCTYPE html>
<style>
#container {
  height: 90vh;
  position: fixed;
  top: 50%;
  background-color: gold;
  margin: auto;
  bottom: 0;
}
</style>
<div id="container">
  <div style="height:90vh;width:50px;outline: 2px dotted red;"></div>
</div>
