CSS3 按钮边框动画的实现

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  <span></span>
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body {
  margin: 0;
  padding: 0;
  background-color: #035f3c;
}

a {
  position: absolute;
  : 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  -webkit-transform: translate(-50%, -50%);
  -moz-transform: translate(-50%, -50%);
  -ms-transform: translate(-50%, -50%);
  -o-transform: translate(-50%, -50%);
  color: #16f03a;
  padding: 30px 60px;
  font-size: 30px;
  letter-spacing: 2px;
  text-transform: uppercase;
  text-decoration: none;
  box-shadow: 0 20px 50px rgba(0, 0, 0, 0.5);
  / to delete length of animation lines: /
  overflow: hidden;
}

a:before {
  content: "";
  position: absolute;
  : 2px;
  left: 2px;
  bottom: 2px;
  width: 50%;
  background: rgba(255, 255, 255, 0.05);
}

a span:nth-child(1) {
  position: absolute;
  : 0;
  left: 0;
  width: 100%;
  height: 2px;
  background: linear-gradient(to right, #035f3c, #16f03a);
  animation: animate1 2s linear infinite;
  -webkit-animation: animate1 2s linear infinite;
}

@keyframes animate1 {
  0% {
    transform: translateX(-100%);
    -webkit-transform: translateX(-100%);
    -moz-transform: translateX(-100%);
    -ms-transform: translateX(-100%);
    -o-transform: translateX(-100%);
  }
  100% {
    transform: translateX(100%);
    -webkit-transform: translateX(100%);
    -moz-transform: translateX(100%);
    -ms-transform: translateX(100%);
    -o-transform: translateX(100%);
  }
}

a span:nth-child(2) {
  position: absolute;
  : 0;
  right: 0;
  width: 2px;
  height: 100%;
  background: linear-gradient(to bottom, #035f3c, #16f03a);
  animation: animate2 2s linear infinite;
  -webkit-animation: animate2 2s linear infinite;
  / add delay to have continuity on effect/
  animation-delay: 1s;
}

@keyframes animate2 {
  0% {
    transform: translateY(-100%);
    -webkit-transform: translateY(-100%);
    -moz-transform: translateY(-100%);
    -ms-transform: translateY(-100%);
    -o-transform: translateY(-100%);
  }
  100% {
    transform: translateY(100%);
    -webkit-transform: translateX(100%);
    -moz-transform: translateX(100%);
    -ms-transform: translateX(100%);
    -o-transform: translateX(100%);
  }
}

a span:nth-child(3) {
  position: absolute;
  bottom: 0;
  right: 0;
  width: 100%;
  height: 2px;
  background: linear-gradient(to left, #035f3c, #16f03a);
  animation: animate3 2s linear infinite;
  -webkit-animation: animate3 2s linear infinite;
}

@keyframes animate3 {
  0% {
    transform: translateX(100%);
    -webkit-transform: translateX(100%);
    -moz-transform: translateX(100%);
    -ms-transform: translateX(100%);
    -o-transform: translateX(100%);
  }
  100% {
    transform: translateX(-100%);
    -webkit-transform: translateX(-100%);
    -moz-transform: translateX(-100%);
    -ms-transform: translateX(-100%);
    -o-transform: translateX(-100%);
  }
}

a span:nth-child(4) {
  position: absolute;
  : 0;
  left: 0;
  width: 2px;
  height: 100%;
  background: linear-gradient(to , #035f3c, #16f03a);
  animation: animate4 2s linear infinite;
  -webkit-animation: animate4 2s linear infinite;
  / add delay to have continuity on effect/
  animation-delay: 1s;
}

@keyframes animate4 {
  0% {
    transform: translateY(100%);
    -webkit-transform: translateY(100%);
    -moz-transform: translateY(100%);
    -ms-transform: translateY(100%);
    -o-transform: translateY(100%);
  }
  100% {
    transform: translateY(-100%);
    -webkit-transform: translateY(-100%);
    -moz-transform: translateY(-100%);
    -ms-transform: translateY(-100%);
    -o-transform: translateY(-100%);
  }
}