While playing around with a bunch of the block editor components, I noticed that many of them don't include reset styles for the fieldset element, and instead expect that common.css's resets will always be available. After chatting about it with @youknowriad, there was the suggestion to update the block editor components with the missing resets, so that they can be responsible for their own needs.

For example, if we switch off or bypass some of the admin CSS we can see that the spacing controls' fieldset displays unexpected borders:

Components to check or update

• block-lock/modal.js
• block-variation-transforms/index.js
• border-radius-control/index.js
• border-height-control/index.js
• link-control/settings.js
• responsive-block-control/index.js (unused, so let's leave it alone for now)
• spacing-sizes-control/index.js
• layouts/grid.js
• layouts/grid.js (separate instance)
• preferences-modal-section/index.js