So I'm working on a tic tac toe game but for some reason my divs won't change their height.

```css
html {
  background-color:black;
  color:white;
  text-align:center;
}

.cell {
  border: 1px solid white;
  margin:1px;
  width:30%;height:30%;
}
```

```html
<header>Tic Tac Toe</header>
<div id='board'>
  <div class='cell'></div>
  <div class='cell'></div>
  <div class='cell'></div>
</div>
```

The width of the divs is changing fine, but as for the height (which is supposed to be 30% of the screen each) are practically lines.

EDIT: This probably isn't necessary but I feel bad, if you do help out, thank you for taking your time. :)