Block editor: as of May 2021, only site and template editors managed to get IFRAME-d, see https://github.com/WordPress/gutenberg/pull/31873#issuecomment-848933961 by @ellatrix

Until post editor gets IFRAME-d and properly isolated from wp-admin, some advanced design systems, such as based on web components, have great trouble getting loaded in post editor, as they would need extra setup and modification just to avoid touching wp-admin chrome.

Unfortunate.