title: Calculate dynamic height with CSS
body: <p>I have a container and an element withing it. </p>
  
  <pre><code>&lt;div id="container"&gt;
      &lt;div class="myEm"&gt;
          ...
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I need to assign width and hight to that element. It should be a square. I can calculate the width relative to the parent container, but how do I pass the same value to the height:</p>
  
  <pre><code>#container .myEm {
     width: calc(100% - 20px);
     height: ???
  }
  </code></pre>