#board {
  display: block;
  position: relative;
  margin: auto;
  background-color: aliceblue;
  overflow: visible;
  width: 50%;
  height: 10%; }

#header {
  display: block;
  margin-bottom: 50px;
  font-size: 5em; }

.cell {
  width: calc(100vw / 40);
  height: calc(100vw / 40); }

.button {
  display: block; }