.btn {
    position: relative;
    display: block;
    margin: 200px auto;
    width: 200px;
    padding: 10px 20px;
    font-size: 20px;
    background: #fff;
    color: #6bdf4e;
    border: 1px solid #6bdf4e;
    cursor: pointer;
}
.btn::after {
    content: attr(data-tip);
    display: none;
    position: absolute;
    padding: 5px 10px;
    left: 50%;
    bottom: 100%;
    margin-bottom: 12px;
    transform: translateX(-50%);
    font-size: 16px;
    background: #000;
    color: #fff;
    cursor: default;
}
.btn::before {
    content: " ";
    position: absolute;
    display: none;
    left: 50%;
    bottom: 100%;
    transform: translateX(-50%);
    margin-bottom: 3px;
    width: 0;
    height: 0;
    border-left: 6px solid transparent;
    border-right: 6px solid transparent;
    border-top: 9px solid #000;
}
.btn:hover::after,
.btn:hover::before {
    display: block;
}