body {
  text-align: center;
}

td {
  width: 30px;
  height: 30px;
}

td > button {
  display: block;
  width: 100%;
  height: 100%;
  font-size: 0.6rem;
}

td {
  border: solid #000000 1px;
  text-align: center;
  font-family: Consolas, monospace;
  font-weight: bold;
}

table {
  margin: 10px auto;
  border-collapse: collapse;
}