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: 
1.20.x
Introduced in version: 
1.20.0
Impacts: 
Module developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/324