<!DOCTYPE html> <!-- Any copyright is dedicated to the Public Domain. - http://creativecommons.org/publicdomain/zero/1.0/ --> <html> <style> summary { display: table-cell; } </style> <body> <details open> <p>This is the details.</p> <!-- Make summary the second element child so that layout will try to render it as the first child. --> <summary>Summary <div>Block in summary</div> </summary> </details> </body> </html>