indigoxela
commented
2 years ago I belief, the original issue might help to clarify.
Drupal 8 completely switched from collapsible fieldsets to details elements, but that caused several problems with (backwards) compatibility. So in Backdrop we decided to add the new form element, but to not replace any existing ones (yet).
Fieldsets are still supposed to get widely used, but if new collapsible form items get added, it's recommended to use details instead wherever possible. The reason behind that: collapsible fieldsets are supposed to be a "Drupalism" and details seem to be better re A11Y.
bugfolder
This comment https://github.com/backdrop/backdrop-issues/issues/4556#issuecomment-1075077513 suggests that the FAPI docs could use some guidance on when to use a fieldset https://docs.backdropcms.org/form_api#fieldset versus using the new details element introduced in 1.20 https://docs.backdropcms.org/form_api#details.
If folks could chime in here on what they think we should say (and we get something of a consensus), I'll merge that information into the FAPI description of both elements.