button::after {
    content: "";
    position: absolute;
    bottom: 0; /* Position underline at the bottom */
    left: 50%; /* Start the underline in the middle */
    transform: translateX(-50%); /* Center it horizontally */
    width: 0; /* Adjust width relative to the button's content */
    height: 4px;
    background-color: #007bff; /* Blue underline */
    border-radius: 2px; /* Optional: Rounded corners for the rectangle */
}

button.selected::after {
    width: 90%; /* Final width when the button is selected */
    transition: width 0.3s ease; /* Smooth animation for width */
}