title: superscript <sup> tag not working when display set to flex
body: <p>I created this div:</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>.content {
    display: flex;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="content"&gt;
    &lt;div&gt;MC&lt;/div&gt;
    &lt;div&gt;MR&lt;/div&gt;
    &lt;div&gt;M+&lt;/div&gt;
    &lt;div&gt;M-&lt;/div&gt;
    &lt;div&gt;MS&lt;/div&gt;
    &lt;div class="sup"&gt;M&lt;sup&gt;-&lt;/sup&gt;&lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>and set the display value for class content to flex, but the sup tag won't work anymore! How can I fix this problem for this tag to work?</p>