#spacer {
  height: 200px;
}

.btn {
  text-decoration: none;
  position: relative;
  overflow: hidden;
  transition: color 0.3s ease;
}

/* Hoặc Cách 2: border-bottom */

.btn::after {
  content: '';
  position: absolute;
  width: 0;
  height: 3px;
  background-color: currentColor;
  bottom: 0;
  left: 0;
  transition: width 0.3s ease-out;
}

.btn:hover::after {
  width: 100%;
}