By default, Drupal 8's "Basic HTML" text format WYSIWYG editor configuration allows content authors to utilize HTML heading elements "h2" through "h5". 

But what if you want to limit authors to a smaller set of block-level HTML elements? In previous versions of Drupal, this would require separate configuraiton of both the text format and the WYSIWYG editor. In Drupal 8, the solution is much simpler - simply limit the text format to use the block-level elements to be allowed (in this case, h4, h5, h6 - the "id" indicates that HTML "id" attributes may be specified on these elements as well):

The WYSIWYG editor automatically displays only those that are allowed by the updated text format!