Correct:
@charset "utf-8";    FAILPASS

Wrong Capitalization:
@CHARSET "utf-8";    PASSFAIL
@ChArSeT "utf-8";    PASSFAIL
@cHaRsEt "utf-8";    PASSFAIL

Wrong Whitespace:
@charset"utf-8";     PASSFAIL
@charset  "utf-8";   PASSFAIL
@charset\9"utf-8";   PASSFAIL
@charset\A"utf-8";   PASSFAIL
@charset\C"utf-8";   PASSFAIL
@charset\D"utf-8";   PASSFAIL
@charset "utf-8" ;   PASSFAIL
 @charset "utf-8";   PASSFAIL

Wrong Quote character:
@charset 'utf-8';    PASSFAIL
@charset "utf-8';    PASSFAIL
@charset 'utf-8";    PASSFAIL

Wrong Semicolon:
@charset "utf-8"     PASSFAIL