<!DOCTYPE html>
<!--
 | When absolutely positioned elements have both left and right set to non-auto,
 | the element's width should be determined by them.
 -->
<html>
<head>
  <style>
    body {
      margin: 0;
    }
    .outer-absolute-block {
      position: absolute;
      width: 300px;
      height: 300px;
      background-color: rgb(255, 100, 100);
    }
    .inner-absolute-block {
      position: absolute;
      height: 50px;
      left: 100px;
      right: 100px;
      top: 0px;
      background-color: rgb(100, 100, 255);
    }
  </style>
</head>

<body>
  <div class="outer-absolute-block">
    <div class="inner-absolute-block"></div>
  </div>
</body>
</html>