<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Reference: Opacity Flattening for Large Images</title>
<style>
div {
  display: inline-block;
  height: 40px;
  width: 40px;
  opacity: 0.25;
}

.blackcolor {
  background: black;
}
</style>
</head>

<body>
<div class="blackcolor"></div>
<div class="blackcolor"></div>
<div class="blackcolor"></div>
<div class="blackcolor"></div>
<div class="blackcolor"></div>
</body>
</html>