如何創(chuàng)建像下圖這樣的多色邊框？

你可以不用偽元素，只用邊界-圖像:線性-漸變

.fancy-border {
  width: 150px;
  height: 150px;
  text-align:center;
  border-top: 5px solid;
  border-image:   linear-gradient(to right, grey 25%, yellow 25%, yellow 50%,red 50%, red 75%, teal 75%) 5;
}

<div class="fancy-border">
  my content
</div>

您可以使用:after或:before偽元素和css線性漸變來實(shí)現(xiàn)，如下所示:

body {
  background: #ccc;
}

.box {
  text-align: center;
  position: relative;
  line-height: 100px;
  background: #fff;
  height: 100px;
  width: 300px;
}

.box:after {
  background: linear-gradient(to right, #bcbcbc 25%,#ffcd02 25%, #ffcd02 50%, #e84f47 50%, #e84f47 75%, #65c1ac 75%);
  position: absolute;
  content: '';
  height: 4px;
  right: 0;
  left: 0;
  top: 0;
}

<div class="box">Div</div>

試試看。

.test {
  width: 500px;
  height: 100px;
  background-color: #ccc;
  position: relative;
}

.test:before,
.test:after {
  content: "";
  position: absolute;
  left: 0px;
  right: 0px;
  height: 10px;
  background-image: -webkit-linear-gradient(0deg, red 20px, blue 20px, blue 40px, yellow 40px, yellow 60px, green 60px, green 80px);
  background-image: -ms-linear-gradient(0deg, red 20px, blue 20px, blue 40px, yellow 40px, yellow 60px, green 60px, green 80px);
  background-size: 80px;
}

.test:before {
  top: 0px;
}

.test:after {
  bottom: 0px;
}

<div class="test"></div>