流动的边框用css怎么实现

很简单，纯css代码就能实现

彩色的流动边框

.box {
  --border-size: 3px;
  --border-angle: 0turn;
  width: 60vmin;
  height: 50vmin;
  background-image: conic-gradient(from var(--border-angle), #213, #112 50%, #213), conic-gradient(from var(--border-angle), transparent 20%, #08f, #f03);
  background-size: calc(100% - (var(--border-size) * 2)) calc(100% - (var(--border-size) * 2)), cover;
  background-position: center center;
  background-repeat: no-repeat;
  -webkit-animation: bg-spin 3s linear infinite;
          animation: bg-spin 3s linear infinite;
}
@-webkit-keyframes bg-spin {
  to {
    --border-angle: 1turn;
  }
}
@keyframes bg-spin {
  to {
    --border-angle: 1turn;
  }
}
.box:hover {
  -webkit-animation-play-state: paused;
          animation-play-state: paused;
}

@property --border-angle {
  syntax: "";
  inherits: true;
  initial-value: 0turn;
}

源码下载 预览效果