Details

HTML Element

elements.css

Render Element

details.html.twig (Starterkit Theme)

Used in forms.

Admin pages

Status report

The status report page has several details html elements, but does not use the details render element. 

<details class="system-status-report__entry" open="" data-once="details">
                    <summary class="system-status-report__status-title system-status-report__status-icon system-status-report__status-icon--error" role="button">
                        File system
          <span class="summary"></span></summary>
          <div class="system-status-report__entry__value">
            Writable (<em>public</em> download method)
                          <div class="description">The directory <em class="placeholder">../private/score</em> does not exist. You may need to set the correct directory at the <a href="/admin/config/media/file-system">file system settings page</a> or change the current directory's permissions so that it is writable.</div>
                      </div>
        </details>

There are core admin styles included on the page.