The new checkbox style provides CSS rules require that all checkboxes come with a "label" element.
If no label was provided, the checkbox will not be visible at all and functionality will be lost.

It has been already observed in core and some apps that checkboxes were broken and could be fixed manually by inserting a label. However, it is not guaranteed that all third party apps will have a label, so there is a risk of breaking third party apps (and potentially other apps under owncloud/apps).

See https://github.com/owncloud/core/pull/18735/files#r40192896

I suggest to add an additional CSS class for the checkbox class to make the new style optional/transitional. Once all/most third party apps have been migrated to work with the new style, that CSS class can be removed.

@Henni I know that you provided another fallback already, but it cannot work for dynamically created checkboxes, so won't cover all cases.

@Henni @jancborchardt @karlitschek @DeepDiver1975