> That would be next level corp spyware Would it be? I was implementing similar stuff for a crappy PHP forum 15+ years ago that wanted to track which users wer

> That would be next level corp spyware Would it be? I was implementing similar stuff for a crappy PHP forum 15+ years ago that wanted to track which users were sharing private posts outside their community. It started with screenshot-tracking by embedding small 1xN base64 images that were almost completely transparent except for some tinting on each pixel to represent a 1 or a 0 bit at the edges of several elements where they wouldn't be noticed. That didn't survive jpeg compression or photos, so I added text transforms that represented tracking bits as well. Things like one vs two spaces after a period or an expanded contraction would correspond to a bit of the ID of the user the text was served to. That worked fine since the site was mostly prose, but I couldn't help but think about generalizing it to encode tracking bits as page styling information as well. Never ended up finishing that implementation because I got too in the weeds trying to account for rendering differences in customized user agents by adding error correction bits to the IDs. That way it wouldn't matter if a few styles were overridden or obscured. I don't think any of these ideas were particularly novel or difficult to think of, so I'd expect plenty of independent implementations over the years. If they do their job right you'd never know about them, so there's likely more than you expect.