A new Form API details element has been added

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,
);
?>