There's two approaches Hyperclay takes.
1. Hosted: You get a bunch of "HTML Apps" that persist themselves by calling their own /save endpoint. We grab the HTML and overwrite their-app-name.html, making a backup/version along the way. (Each user can edit their own app only, but they can also enable signups so that other people can fork their app. We also have plans to allow them to ship optional updates to forked apps.)
2. Local: You download the open-source Hyperclay Local [0] and you can have your own personal, local HTML apps that also call the /save endpoint and make backups. You're also open to extracting the core code from this to host your own personally malleable apps on your own server (just implement some kind of auth)
So… it's a server that stores HTML files?
Sounds like modern version which replaces FTP access with nodejs server. But you have to host it on a server or pay monthly fee anyway so you do need a server. AKA ad for OPs business.
And probably uses the Mutation Observer to capture DOM changes
https://developer.mozilla.org/en-US/docs/Web/API/MutationObs...
Why are you trying so hard to avoid saying that it uses a NodeJS server?