The html_tag type allows you to easily add arbitrary HTML tags within the Form API structure. Where you would have to use the markup type in Drupal 7, you will specify your own HTML in the text in Drupal 8; this allows you to easily separate out the tag from the content. Take into consideration the following example:

$form['tag'] = array( 
  '#type' => 'html_tag', 
  '#tag' => 'h1', 
  '#value' => $this->t('Mastering Drupal 8'), 
);