.required {
  color: red;
}

input:required:invalid,
input:focus:invalid {
  background-image: url(../images/reject.png);
  background-position: right top;
  background-repeat: no-repeat;
  background-size: 1.4rem;
}

input:required:valid {
  background-image: url(../images/correct.png);
  background-position: right top;
  background-repeat: no-repeat;
  background-size: 1.4rem;
}