<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>CSS Masking: mask-size: mask layer size</title>
<link rel="author" title="Astley Chen" href="mailto:aschen@mozilla.com">
<link rel="author" title="Mozilla" href="https://www.mozilla.org">
<style type="text/css">
#outer {
border: 10px solid black;
width: 64px;
height: 128px;
}
#inner {
width: 64px;
height: 64px;
background-color: purple;
}
</style>
</head>
<body>
<div id="outer">
<div id="inner"></div>
</div>
</body>
</html>