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