The HTTP header attempts to set the character encoding to ISO 8859-15. The page contains an encoding declaration in a meta charset attribute that attempts to set the character encoding to ISO 8859-1.

The test contains a div with a class name that contains the following sequence of bytes: 0xC3 0xBD 0xC3 0xA4 0xC3 0xA8. These represent different sequences of characters in ISO 8859-15, ISO 8859-1 and UTF-8. The external, UTF-8-encoded stylesheet contains a selector .test div.ÜÀÚ. This matches the sequence of bytes above when they are interpreted as ISO 8859-15. If the class name matches the selector then the test will pass.