Fieldsets lack indentation

I'm not sure why this would be a problem: the primary purpose is to remove extra space.

You seem to mix two things here. This specific toolset is, in fact, a dependent fieldset that opens up when the upper checkbox is checked. But this is not a general attributes of fieldsets, this is something specific to this Devel form. If we changed all fieldsets the way you suggest, they would become larger anywhere in the UI, not just in this dependent position.

Thanks for a fast reply, I guess the fieldset element as a side effect of the border and text indentation acts a hierarchy signifier. I'll move the issue to Drupal core, to clarify how to best indicate hierarchy between form elements.

I would, frankly, expect Devel to style their form to indent the dependent fieldset in this particular case.

I think it would be better to find a universal solution, for cases such as this. Maybe we can add styling to dependent fields and similar "sub-fields", to indent them?

I can't find anything in the classes that would help us tell apart this case, so it would be a task for the core to add something that we can rely on.

Thanks for investigating, maybe we need a new element, to indicate hierarchy?

I never looked into the code that generates these form elements, so I really don't know. This is just a shot in the dark, but maybe a class of form-indent-level-N would be beneficial.