body {
  box-sizing: border-box;
}

.board {
  width: 300px;
  border: 2px solid purple;
}

.row {
  display: flex;
}
.cell {
  width: 100px;
  height: 100px;
  border: 1px solid green;
  text-align: center;
  line-height: 50px;
}