When I was running a newer site through the CSS3 validation test, I received the following, cryptic error message:

This profile has a very specific syntax for @charset: @charset followed by exactly one space, followed by the name of the encoding in quotes, followed immediately by a semicolon. @charset ‘utf-8′;

The line I had been using was exactly as stated : @charset ‘utf-8′;
Once I changed it to uppercase and double-quotes it validated : @charset “UTF-8″;

Go figure.