https://www.davidrumsey.com/ has had on-line georeferencing for years. I have no idea how it's done. I used it for a while until I found QGIS.
Here's a link to a page open to georeferencing. I'm logged in so the link may not work, but one can create an account. Although I did that five years ago at least.
A bunch of jS, but nothingPS I recognized, but thenjust saw your comment. I'm not very knowledgeablejust saying it can be done. Maybe Ramsey can help you. From their about page: Georeferencer v4 software and hosting by Klokan Technologies