<!DOCTYPE html>
<!--
 | Opacity should affect the appearance of all children of a container node.
 |   https://www.w3.org/TR/css3-color/#transparency
 -->
<html>
<head>
  <style>
    body {
      opacity: 0.5;
    }
    div {
      width: 100px;
      height: 50px;
    }
    .red {
      background-color: rgb(255, 0, 0);
    }
    .green {
      background-color: rgb(0, 255, 0);
    }
    .blue {
      background-color: rgb(0, 0, 255);
    }
  </style>
</head>
<body>
  <div class="red"></div>
  <div class="green"></div>
  <div class="blue"></div>
</body>
</html>