<a href="#">Hello</a>

a {
  position: relative;
  color: #000;
  padding: 5px;
  text-decoration: none;
  &:after {
    content: "";
    position: absolute;
    width: 100%;
    height: 5px;
    bottom: 0;
    left: 0;
    background-color: red;
    animation: line 2s infinite linear; /*infinite - можно убрать не будет повтора*/
  }
}

@keyframes line {
0% {width: 0;}
100% {width: 100%;}
}