title: HTML Can't Change Height of Div
body: <p>So I'm working on a tic tac toe game but for some reason my divs won't change their height.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>html {
    background-color:black;
    color:white;
    text-align:center;
  }
  
  .cell {
    border: 1px solid white;
    margin:1px;
    width:30%;height:30%;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;header&gt;Tic Tac Toe&lt;/header&gt;
  &lt;div id='board'&gt;
    &lt;div class='cell'&gt;&lt;/div&gt;
    &lt;div class='cell'&gt;&lt;/div&gt;
    &lt;div class='cell'&gt;&lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>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.</p>
  
  <p>EDIT: This probably isn't necessary but I feel bad, if you do help out, thank you for taking your time. :)</p>