按钮模版：btn-2

效果展示

实现思路

为了动画初始化的时候，四条边不会同时显示，通过绝对定位隐藏其他三条边

使用了 @keyframes 定义了四个不同方向上的动画变化效果：toTop、toBottom、toLeft、toRight

每个 <span> 标签代表按钮中的一条边，分别使用了 nth-child 来定位，并分别应用了不同的动画

这个动画的核心便是动画延迟的应用，整个动画的时长是 1s，那么每个动画的延迟相差 0.25s 是最优解

然后安装动画顺序进行叠加就可以了

实现代码

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<button class="btn-2">
  <span></span>
  <span></span>
  <span></span>
  <span></span>
  BTN-2
</button>
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@keyframes toTop {
  0% {
    bottom: -100%;
  }
  50%,
  100% {
    bottom: 100%;
  }
}
@keyframes toBottom {
  0% {
    top: -100%;
  }
  50%,
  100% {
    top: 100%;
  }
}
@keyframes toLeft {
  0% {
    right: -100%;
  }
  50%,
  100% {
    right: 100%;
  }
}
@keyframes toRight {
  0% {
    left: -100%;
  }
  50%,
  100% {
    left: 100%;
  }
}
.btn-2 {
  position: relative;
  padding: 16px 28px;
  color: #03e9f4;
  background: transparent;
  letter-spacing: 4px;
  overflow: hidden;
  transition: 0.5s;
  border: none;
  cursor: pointer;
}
.btn-2:hover {
  background: #03e9f4;
  color: #000;
  box-shadow: 0 0 5px #03e9f4,
    0 0 25px #03e9f4,
    0 0 50px #03e9f4,
    0 0 200px #03e9f4;
}
.btn-2 span {
  position: absolute;
  display: block;
}
.btn-2 span:nth-child(1) {
  top: 0;
  left: 0;
  width: 100%;
  height: 2px;
  background: linear-gradient(90deg, transparent, #03e9f4);
  animation: toRight 1s linear infinite;
}
.btn-2 span:nth-child(2) {
  top: -100%;
  right: 0;
  width: 2px;
  height: 100%;
  background: linear-gradient(180deg, transparent, #03e9f4);
  animation: toBottom 1s linear infinite;
  animation-delay: 0.25s;
}
.btn-2 span:nth-child(3) {
  bottom: 0;
  right: -100%;
  width: 100%;
  height: 2px;
  background: linear-gradient(270deg, transparent, #03e9f4);
  animation: toLeft 1s linear infinite;
  animation-delay: 0.50s;
}
.btn-2 span:nth-child(4) {
  bottom: -100%;
  left: 0;
  width: 2px;
  height: 100%;
  background: linear-gradient(360deg, transparent, #03e9f4);
  animation: toTop 1s linear infinite;
  animation-delay: 0.75s;
}