The target page is served with no character encoding declarations in HTTP or inside the document other than a UTF-16 big-endian BOM.

The test on the target page contains a div with a class name that contains a sequence of bytes that only reads as ÜÀÚ if the character encoding is UTF-16BE. The external, UTF-8-encoded stylesheet contains a selector .test div.ÜÀÚ. This only matches the class name in the html if sequence of bytes in the html page is interpreted as UTF-16BE. If the class name matches the selector then the top box will turn green.