#backtop {
    /* (A) BOTTOM RIGHT CORNER */
    position: fixed;
    bottom: 20px;
    right: 20px;
    z-index: 999;

    /* (B) HIDDEN BY DEFAULT */
    display: none;
    visibility: none;
    opacity: 0;

    /* (C) COSMETICS */
    cursor: pointer;
    transition: opacity 0.3s;
}

/* (D) SHOW BUTTON */
#backtop.show {
    display: flex;
    visibility: visible;
    opacity: 1;
}

/* (X) DOES NOT MATTER */
* {
    box-sizing: border-box;
}