The Gutenberg Handbook mentions that some CSS for WordPress core blocks are applied to the front-end and within the editor by default (and that they cannot be disabled) but there are more opinionated CSS styles available that can be optionally added by adding the following your theme (in one of the PHP files, typically done through functions.php).
add_theme_support( 'wp-block-styles' );
I'm trying to find a list of these CSS rules added by
I've searched through the Gutenberg repo and the only contents of
wp-block-styles are the documentation that I linked to above.