HTML Can't Change Height of Div

Answer 1

That is because you don't have a height to the board and the 30% of almost 0 is... 0.

Add some height to the div with the id of board.

Add this to your css:

html {
  background-color:black;
  color:white;
  text-align:center;
}
#board{
 height:300px;
}
.cell {
  border: 1px solid white;
  margin:1px;
  width:30%;height:30%;
}