请问这种动画效果用CSS该怎么做？

Question

如题，不涉及js的话觉得不好做啊

上传的图片看着不怎么清晰，也就是鼠标放上去的时候有一条线围绕矩形走一圈，鼠标离开后就会退回到原点。

Answer 1

https://jsbin.com/xeluqil/2/edit?css,output

  <a href="#" class="btn-hover">
    现在购买
    <span></span>
    <span></span>
    <span></span>
    <span></span>
  </a>

.btn-hover {
  display: inline-block;
  box-sizing: border-box;
  width: 120px;
  height: 50px;
  background: #ddd;
  line-height: 50px;
  text-align: center;
  text-decoration: none;
  color: #aaa;
  transition: all .5s ease;
  position: relative;
}

.btn-hover:before {
  content: " ";
  position: absolute;
  box-sizing: border-box;
  width: 100%;
  height: 100%;
  border: 2px solid #ccc;
  left: 0;
  top: 0;
}
.btn-hover:hover {
  color: #06f;
}

.btn-hover > span {
  position: absolute;
  background: #06f;
  transition: all .3s linear;
}

.btn-hover:hover > span:nth-child(1),
.btn-hover:hover > span:nth-child(3) {
  width: 100%;
}
.btn-hover:hover > span:nth-child(2),
.btn-hover:hover > span:nth-child(4) {
  height: 100%;
}
.btn-hover > span:nth-child(1) {
  top: 0;
  left: 0;
  width: 0%;
  height: 2px;
  transition-delay: .9s;
}
.btn-hover > span:nth-child(2) {
  top: 0;
  right: 0;
  width: 2px;
  height: 0%;
  transition-delay: .6s;
}
.btn-hover > span:nth-child(3) {
  bottom: 0;
  right: 0;
  width: 0%;
  height: 2px;
  transition-delay: .3s;
}
.btn-hover > span:nth-child(4) {
  bottom: 0;
  left: 0;
  width: 2px;
  height: 0%;
  transition-delay: 0s;
}

.btn-hover:hover > span:nth-child(1) { transition-delay: 0s; }
.btn-hover:hover > span:nth-child(2) { transition-delay: .3s; }
.btn-hover:hover > span:nth-child(3) { transition-delay: .6s; }
.btn-hover:hover > span:nth-child(4) { transition-delay: .9s; }