body {
    width: 100%;
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
}
#challenge20 a {
    text-align: center;
    padding: 20px 30px;
    border: 1px solid #ddd;
    background-color: #333;
    color: #fff;
    font-size: 25px;
    text-decoration: none;
    box-shadow: 0 0 5px #3333331c;
    font-family: 'Lucida Sans', 'Lucida Sans Regular', 'Lucida Grande', 'Lucida Sans Unicode', Geneva, Verdana, sans-serif;
    position: relative;
    transition: 0.5s;
    overflow: hidden;
}
#challenge20 a::after {
    content: '';
    position: absolute;
    top: 0;
    margin-left: 50px;
    width: 10%;
    height: 100%;
    transform: skew(20deg);
    background-color: #ffffff36;
    transition: 1.0s;
}
#challenge20 a:hover:after {
    margin-left: -500px;
}
#challenge20 a:hover{
    background-color: #000;
}