Without fanfare, Apple recently updated iCloud.com to allow users to restore files, contacts, and/or calendars and reminders hosted on the service, opening up a recovery window for mistakes.
The options are hidden under the Settings app on the website. In each case, different limits apply. Deleted files for example are kept on Apple's servers for varying amounts of time, though apparently no more than 30 days. Recovered files appear in iCloud Drive.
Contacts can only be restored as an archive, not individually, and doing so will revert contacts across every device signed into the same iCloud account. This creates an archive of the latest contact data though, making it possible to undo the restore.
A similar system is in effect for calendars and reminders. Apple cautions that all sharing will be stripped out during a restore, forcing users to once again offer or ask for permissions. Any scheduled events will be canceled and recreated, which could cause confusion as invited guests will see both cancellation notices and new invitations.
Because they weren't announced it's not clear when the restore options were added, although they appear to have surfaced sometime this week.
Apple has been gradually developing iCloud.com into a comprehensive platform, one major addition in the past year being iCloud Photo Library. The Web versions of Pages, Numbers, and Keynote are still in beta however, despite having been online since 2013.