title: Same width as height on a flex child element
body: <p>I'm working on a navbar where I will have a burger menu for toggling an extended menu.</p>
  
  <p>I want to avoid to explicitly set a width to the burger menu element (<code>.extendedMenu__toggle</code>) and have it scale based on the font-size set on <code>.header</code> if possible.</p>
  
  <p>The element gets it height from the parent as it's flex, but not sure how I can translate that into a proper width property so that I get a perfect square.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>html {
    font-size: 62.5%;
  }
  
  html, body {
    font-family: helvetica;
    line-height: 1.45;
  }
  
  .header {
    left: 0;
    position: fixed;
    right: 0;
    top: 0;
    z-index: 1000;
  
    font-size: 1.3rem;
    font-weight: bold;
  }
  
  .nav {
    display: flex;
    justify-content: flex-start;
    margin: 0;
    position: relative;
    z-index: 5;
    margin: 1rem;
  }
  
  .extendedMenu__toggle {
    /*width: 37px;
    height: 37px;*/
    background: red;
  }
  
  .navLink {
    padding: 0.75rem 1.25rem;
    text-decoration: none;
    border: 2px solid;
    border-radius: 30px;
    margin-left: 0.75rem;
    text-transform: uppercase;
  }
  
  .navLink:first-child {
    margin-left: 0;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;header class="header" role="banner"&gt;
    &lt;nav class="nav"&gt;
      &lt;div class="extendedMenu__toggle"&gt;
        &lt;span&gt;&lt;/span&gt;
        &lt;span&gt;&lt;/span&gt;
        &lt;span&gt;&lt;/span&gt;
      &lt;/div&gt;
  
      &lt;a class="navLink" href="#"&gt;Home&lt;/a&gt;
      &lt;a class="navLink" href="#"&gt;About&lt;/a&gt;
      &lt;a class="navLink" href="#"&gt;News&lt;/a&gt;
    &lt;/nav&gt;
  &lt;/header&gt;</code></pre>
  </div>
  </div>
  </p>