mozilla-release/layout/reftests/bugs/1316719-1a.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>mask-image with scale transform</title>
<style>

body {
  margin: 0;
}

#transform {
  width: 10px;
  height: 10px;
  transform: scale(20);
  transform-origin: top left;
  padding: 5px 10px;
}

#opacity {
  opacity: 0.8;
}

#mask {
  width: 10px;
  height: 10px;
  background: green;
  mask-image: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewbox="0 0 10 10"><circle cx="5" cy="5" r="5" fill="black"/></svg>');
}

#activator {
  border: 1px solid transparent;
  will-change: transform;
}

</style>

<div id="transform">
  <div id="opacity">
    <div id="mask">
      <div id="activator"></div>
    </div>
  </div>
</div>