In reply to T Piwowar.

Thanks for the feedback!

The Classic Editor uses add_editor_style() to allow themes to add custom CSS to the editor, to ensure what you're writing in the editor looks like what you'll see when you preview your post.

Gutenberg provides a similar mechanism for themes to style blocks in the same way, but it does require the theme to provide those styles.

At the moment, the core Twenty* themes haven't been updated to add block editor styling support, but I agree that at least Twenty Seventeen (and maybe a few of the more recent core themes) should be updated in this way. 🙂