* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
.line{
  position: relative;
  width: 400px;
  height: 2px;
  margin: 200px;
  overflow: hidden;
  transform: rotate(-30deg);
}
.line::after{
  content: '';
  width: 200%;
  position: absolute;
  left: 0;
  top:0;
  bottom:0;
  border-bottom:2px dashed #000;
  animation: 5s line linear infinite;
}
@keyframes line {
  0%{
    transform: translateX(0);
  }
  100%{
    transform: translateX(-50%);
  }
}