@import url("https://fonts.googleapis.com/css?family=Lato:300");
body {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-pack: center;
      -ms-flex-pack: center;
          justify-content: center;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  height: 90vh;
  background: #222;
}

.gradient-border {
  --border-width: 3px;
  position: relative;
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-pack: center;
      -ms-flex-pack: center;
          justify-content: center;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  width: 350px;
  height: 250px;
  font-family: Lato, sans-serif;
  font-size: 2.5rem;
  text-transform: uppercase;
  color: white;
  background: #222;
  border-radius: var(--border-width);
}

.gradient-border::after {
  position: absolute;
  content: "";
  top: calc(-1 * var(--border-width));
  left: calc(-1 * var(--border-width));
  z-index: -1;
  width: calc(100% + var(--border-width) * 2);
  height: calc(100% + var(--border-width) * 2);
  background: linear-gradient(60deg, #5f86f2, #a65ff2, #f25fd0, #f25f61, #f2cb5f, #abf25f, #5ff281, #5ff2f0);
  background-size: 300% 300%;
  background-position: 0 50%;
  border-radius: calc(2 * var(--border-width));
  -webkit-animation: moveGradient 4s alternate infinite;
          animation: moveGradient 4s alternate infinite;
}

@-webkit-keyframes moveGradient {
  50% {
    background-position: 100% 50%;
  }
}

@keyframes moveGradient {
  50% {
    background-position: 100% 50%;
  }
}