例如如图所示边框

.card-middle {
  width: 672px;
  height: 486px;
  border: 1px solid #5fadec;
  border-radius: 5px;
  position: relative;
}

.card-middle::before {
  content: '';
  position: absolute;
  top: -4px;
  left: -4px;
  width: 680px;
  height: 448px;
  border: 25px solid transparent;
  border-image: url(../assets/20171030140854037.png);
  border-image-slice: 34;
  border-image-repeat: round;
}

 用伪类即可