div {
    box-sizing: border-box;
}

.board {
    width: 84vw;
    margin: 10px auto;
    outline: 1px solid black;
}

.board .field {
    border: 1px solid black;
    width: 12vw;
    height: 12vw;
    float: left;
}

.board .row .field:first-child {
    clear: both;
}

.board .field .piece {
    width: 10vw;
    height: 10vw;
    border-radius: 50%;
    margin: 1vw;
}

.board .field .blue {
    background-color: blue;
}

.board .field .red {
    background-color: red;
}