<!doctype html>
<meta charset="utf-8">
<title>CSS Test Reference</title>
<style>
  #container {
    width: 400px;
    height: 400px;
    margin: 100px;
    background-color: purple;
    position: relative;
  }
  #spacer {
    height: 300px;
  }
  #bottom {
    height: 100px;
    background-color: green;
  }
  #abspos {
    position: absolute;
    width: 100px;
    height: 100px;
    background-color: green;
    top: 0;
    left: 0;
  }
</style>
<div id="container">
  <div id="spacer"></div>
  <div id="bottom"></div>
  <div id="abspos"></div>
</div>