Correct:
@charset "utf-8";    PASS

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

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

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

Wrong Semicolon:
@charset "utf-8"     PASS