SOURCE

body:before{
  content: '';
  display: block;
  height: 4px;
  color: black;
  background-size: 21px auto;
  background: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 20 4'%3E%3Cpath fill='none' stroke='green' d='M0 3.5c5 0 5-3 10-3s5 3 10 3 5-3 10-3 5 3 10 3'/%3E%3C/svg%3E") repeat-x 0 100%;
  animation: waveMove 0.5s infinite linear;
}
@keyframes waveMove{
  0% {
    background-position: 0 100%;
  }
  100% {
      background-position: -20px 100%;
  }
}