<!DOCTYPE html>
<style>
#container {
  text-align: center;
  width: 100px;
}
#relpos {
  position: relative;
  background: red;
}
#abspos {
  position: absolute;
  left: 0;
  top: 0;
  width: 50px;
  height: 50px;
  background: green;
}
</style>
<body>
  <div id="container">
    <span id="relpos">x
      <span id="abspos"></span>
    </span>
  </div>
</body>
