<!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"> div { width: 60px; height: 120px; } #outer { border: 10px solid black; } #inner { height: 60px; background-color: purple; } </style> </head> <body> <div id="outer"> <div id="inner"></div> </div> </body> </html>