html,
body {
    width: 100%;
    height: 100%;
    padding: 0;
    margin: 0;
}

body {
    background-color: black;
    display: flex;
    justify-content: center;
    align-items: center;
}

#download {
    display: none;
    position: absolute;
    bottom: 10px;
    right: 10px;
}

#download.visible {
    display: block;
}