States can quickly become complex. Like styles for a logged in user with a 3-years-membership badge or styles for a placeholder that only new users see.

Or styles for a CMS module that is currently not in use (e.g. for open positions in a company).

I wonder what the least used, yet still used CSS is I ever wrote (relative to project traffic). Maybe some feature a Client requested, I knew he would never really use, but he insisted on (deep linking to some modal for a facbeook post or something).