button hover했을 때 아래줄 생기게 하기

button::after {
  content: "";
  width: 0;
  height: 2px;
  background-color: #0066ff;
  position: absolute;
  bottom: 0;
  left: 0;
  transition: width 250ms ease-in;
}

button:hover::after {
  width: 100%;
}