span {
  margin: 5px;
  font-size: 3rem;
  position: relative;
}
span:hover {
  cursor: pointer;
}
span::after {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  border: 1px solid black;
  transform: scalex(0);
  transform-origin: right;
  transition: transform 0.4s;
}

span:hover::after {
  transform: scalex(1);
  transform-origin: left;
}