<!--
     Any copyright is dedicated to the Public Domain.
     http://creativecommons.org/publicdomain/zero/1.0/
-->
<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200">

  <title>Reference for alpha mask</title>

  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=793617 -->

  <rect x="10" y="10" width="80" height="80" fill="blue" fill-opacity="0.5"/>
</svg>