body, html {
    margin: 0;
    padding: 0;
    width: 100vw;
    height: 100vh;
    overflow: hidden;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    background: peachpuff
}

#square {
    background-color: darkcyan;
    aspect-ratio: 1;
    width: 50vmin;
    box-shadow: inset 0 0 0 1vmin black;
}