.br800on {
  display: none;
}

.br480on {
  display: none;
}

/* brをonにする */
@media screen and (max-width: 800px) { 
  #body .br800on {
    display: block;
  }
}

@media screen and (max-width: 480px) { 
  #body .br480on {
    display: block;
  }
}