.class {
  -webkit-border-image: -webkit-linear-gradient(black, white) 20% fill stretch stretch;
  -moz-border-image: -moz-linear-gradient(black, white) 20% fill stretch stretch;
  -o-border-image: -o-linear-gradient(black, white) 20% fill stretch stretch;
  border-image: -webkit-linear-gradient(black, white) 20% fill stretch stretch;
  border-image: -moz-linear-gradient(black, white) 20% fill stretch stretch;
  border-image: -o-linear-gradient(black, white) 20% fill stretch stretch;
  border-image: linear-gradient(black, white) 20% fill stretch stretch;
}