<!DOCTYPE html>
<!--
 | Absolutely positioned elements should be positionable by specifying left
 | as a percentage.
 -->
<html>
<head>
  <style>
    .container {
      position: absolute;
      width: 300px;
      height: 300px;
      background-color: rgb(255, 100, 100);
    }
    .block {
      position: absolute;
      width: 50px;
      height: 50px;
      left: 50%;
      background-color: rgb(100, 100, 255);
    }
  </style>
</head>
<body>
  <div class="container">
    <div class="block"></div>
  </div>
</body>
</html>