Because the CSS is in a different encoding. That's the whole  point of the test. ;-)  

Here are the notes you get in the i18n framework version of this test:

[[
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.ÃœÃ€Ãš. This matches the sequence of bytes above when they are interpreted as ISO 8859-15. If the class name matches the selector then the box will turn green.

The only character encoding declaration for this HTML file is in the HTTP header, which sets the encoding to ISO 8859-15. (This test needs to be run from a server, since otherwise the HTTP information is absent.)
]]