<!DOCTYPE html>
<html>
  <head>
    <style type="text/css">
      div.outer, div.inner, details, summary { border: solid 1px black;}
      details:after, summary:after {
        content: "after";
      }
      details:before, summary:before {
        content: "before";
      }
    </style>
  </head>
  <body>
      <details open>
        <summary>Summary
          <div class="inner">Inner Div</div>
        </summary>
      Details
      </details>
  </body>
</html>