table.sudoku {
  border-collapse: collapse;
  margin: auto;
  font-family: sans-serif;
  font-weight: bold;
  font-size: 28px;
}
.sudoku td {
  border: 1px solid black;
  text-align: center;
  vertical-align: middle;
  width: 40px;
  height: 40px;
}
.sudoku td.br {
  border-right: 3px solid black;
}
.sudoku td.bb {
  border-bottom: 3px solid black;
}