.btnAn {
  position: relative;
  overflow: hidden;
  transition: 0.3s;
}
.btnAn:after {
  position: absolute;
  transition: 0.3s;
  content: "";
  width: 0;
  left: 50%;
  bottom: 4px;
  height: 5px;
  background: #ffd800;
}
.btnAn:hover {
  cursor: pointer;
}
.btnAn:hover:after {
  width: 100%;
  left: 0;
}