<a href="#" class="button instagram"><span class="gradient"></span>Sign Up</a>

body {
  display: flex;
  justify-content: center;
  align-items: center;
}

body {
  width: 100%;
  height: 90vh;
}

.button {
  display: block;
  width: 320px;
  max-width: 100%;
  margin: 0 auto;
  margin-bottom: 0;
  overflow: hidden;
  position: relative;
  transform: translatez(0);
  text-decoration: none;
  box-sizing: border-box;
  font-size: 24px;
  font-weight: normal;
  box-shadow: 0 9px 18px rgba(0,0,0,0.2);
}

.instagram {
  text-align: center;
  border-radius: 50px;
  padding: 26px;
  color: white;
  background: #BD3381;
  transition: all 0.2s ease-out 0s;
}

.gradient {
  display: block;
  position: absolute;
  top: 0;
  right: 0;
  width: 100%;
  height: 100%;
  bottom: auto;
  margin: auto;
  z-index: -1;
  background: radial-gradient(90px circle at top center, rgba(238,88,63,.8) 30%, rgba(255,255,255,0));
  transition: all 0s ease-out 0s;
  transform: translatex(-140px);
  animation: 18s linear 0s infinite move;
}

@keyframes move {
  0% {
    transform: translatex(-140px);
  }
  25% {
    transform: translatex(140px);
    opacity: 0.3;
  }
  50% {
    transform: translatex(140px);
    opacity: 1;
    background: radial-gradient(90px circle at bottom center, rgba(238,88,63,.5) 30%, rgba(255,255,255,0));
  }
  75% {
    transform: translatex(-140px);
    opacity: 0.3;
  }
  100% {
    opacity: 1;
    transform: translatex(-140px);
    background: radial-gradient(90px circle at top center, rgba(238,88,63,.5) 30%, rgba(255,255,255,0));
  }
}