Drupal - How to make custom fieldset collapsed by default in Drupal 8

All collapsible fieldsets have been replaced with HTML5 details elements.

Fieldset and legand are very hard to style and its not recommended to use with Drupal 8 any more. Instead its replaced with more cross browser way, which is the HTML5 Details & Summary tags.

'#type' => 'details'

https://www.drupal.org/node/1852020

Drupal 7

$form['advanced'] = array(
  '#type' => 'fieldset',
  '#title' => t('Advanced settings'),
  '#collapsible' => TRUE,
  '#collapsed' => FALSE,
  '#description' => t('Lorem ipsum.'),
);

Drupal 8

$form['advanced'] = array(
  '#type' => 'details',
  '#title' => t('Advanced settings'),
  '#description' => t('Lorem ipsum.'),
  '#open' => TRUE, // Controls the HTML5 'open' attribute. Defaults to FALSE.
);