main.css

body {
  margin: 0;
  padding: 0;
  background: #0c002b;
  font-family: sans-serif;
}

a {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  color: #1670f0;
  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);
  overflow: hidden;
}
a:before {
  content: "";
  position: absolute;
  top: 2px;
  left: 2px;
  bottom: 2px;
  width: 50%;
  background: rgba(255, 255, 255, 0.05);
}
a span:nth-child(1) {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 2px;
  background: linear-gradient(to right, #0c002b, #1779ff);
  animation: animate1 2s linear infinite;
}
@keyframes animate1 {
  0% {
    transform: translateX(-100%);
  }
  100% {
    transform: translateX(100%);
  }
}
a span:nth-child(2) {
  position: absolute;
  top: 0;
  right: 0;
  width: 2px;
  height: 100%;
  background: linear-gradient(to bottom, #0c002b, #1779ff);
  animation: animate2 2s linear infinite;
  animation-delay: 1s;
}
@keyframes animate2 {
  0% {
    transform: translateY(-100%);
  }
  100% {
    transform: translateY(100%);
  }
}
a span:nth-child(3) {
  position: absolute;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 2px;
  background: linear-gradient(to left, #0c002b, #1779ff);
  animation: animate3 2s linear infinite;
}
@keyframes animate3 {
  0% {
    transform: translateX(100%);
  }
  100% {
    transform: translateX(-100%);
  }
}
a span:nth-child(4) {
  position: absolute;
  top: 0;
  left: 0;
  width: 2px;
  height: 100%;
  background: linear-gradient(to top, #0c002b, #1779ff);
  animation: animate4 2s linear infinite;
  animation-delay: 1s;
}
@keyframes animate4 {
  0% {
    transform: translateY(100%);
  }
  100% {
    transform: translateY(-100%);
  }
}