:root {
    font-size: 3em;
    font-family: sans-serif;
}

html, body {
    height: 100%;
    margin: 0;
    display: grid;
    place-items: center;
}

h1 {
    position: fixed;
    top: 20px;
}

main {
    display: grid;
    place-items: center;
    height: 800px;
    width: 800px;

    & > * {
        display: grid;
        place-items: center;
        height: 100%;
        width: 100%;
        border: 3px solid black;
        user-select: none;
        cursor: pointer;
    }
}

.hidden {
    color: transparent;
}