<!doctype html>
<meta charset="utf-8">
<title>CSS Test Reference</title>
<style>
  .container {
    border: 1px solid green;
    width: 100px;
    height: 100px;
    margin-top: 100px;
    display: flex;
    position: relative;
  }
  .abspos {
    position: absolute;
    top: 0;
    left: 0;
    background: orange;
    height: 20px;
    width: 20px;
  }
</style>
<div class="container">
  <div class="abspos"></div>
</div>