.sudoku-grid {
    display: flex;
    flex-direction: column;
}

.row {
    display: flex;
}

.cell {
    width: 50px;
    height: 50px;
    border-width: 1px 0px 0px 1px;
    border-color: black;
    border-style: solid;
    display: block;
    flex: 0 0 auto;
    padding: 0px;
}

.column-0, .column-3, .column-6 {
    border-left: 3px solid black;
}

.column-8 {
    border-right: 3px solid black;
}

.row-0, .row-3, .row-6 {
    border-top: 3px solid black;
}

.row-8 {
    border-bottom: 3px solid black;
}

span {
    display: block;
    font: 42px Arial, sans-serif;
    text-align: center;
    margin: 0px;
    height: 44px;
    position: relative;
}

.rules {
    width: 466px;
    text-align: justify;
}