<!DOCTYPE html>
<meta charset="utf-8">
<title>Masks need to cause grouping / intermediate surfaces</title>
<style>

.outer {
  filter: opacity(50%);
}

.box {
  width: 100px;
  height: 100px;
}

.one {
  background: green;
}

.another {
  background: blue;
  margin-top: -50px;
  margin-left: 50px;
}

</style>

<div class="outer">
  <div class="one box"></div>
  <div class="another box"></div>
</div>