HTML5 adds support for a <details> element. The element creates a disclosure widget in which information is visible only when the widget is toggled into an "open" state. A summary or label must be provided using the <summary> element.

Backdrop currently uses the HTML <fieldset> element with custom JavaScript, to achieve a similar result. The <details> element has native browser support, without the need for JavaScript.

Examples: Before: <?php$form['fieldset'] = array(  '#type' => 'fieldset',  '#title' => t('Summary'),  '#description' => t('Longer description within.'),  '#collapsible' => TRUE,  '#collapsed' => TRUE,);?> After: <?php$form['details'] = array(  '#type' => 'details',  '#summary' => t('Summary'),  '#details' => t('Longer description within.'),  '#open' => TRUE,);?>
Introduced in branch: 
1.x
Introduced in version: 
1.20.0
Impacts: 
Module developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/324