Emacs doesn't recognize "utf8" as a valid coding system. The correct
name is "utf-8". This is in line with the UTF-8 FAQ:

  http://www.cl.cam.ac.uk/~mgk25/unicode.html#utf-8

  The official name and spelling of this encoding is UTF-8, where UTF
  stands for UCS Transformation Format. Please do not write UTF-8 in
  any documentation text in other ways (such as utf8 or UTF_8), unless
  of course you refer to a variable name and not the encoding itself.