Quoting the mega Gutenberg list by kdiamant:

We need to fix blocks styling in the editor.
I think bootstrap breaks wordpress gutenberg editor styling.
There is always the 'preview' mode where editors can see how the page would look like.

Adding some notes:

• Loading the frontend styles in the editor breaks some forms styles because of the `_forms` custom styling for controls in the `styleguide`.
• Look for global or pure element CSS selectors, like `body`, `html`, `a`, etc.
• Ideally, each block's style should be isolated, and avoid inheriting styles from the master-theme.
• As for Bootstrap, maybe we can just load the layout parts, which are the only parts I think we are really using from it. We don't use bootstrap buttons or other cosmetic styles.