A guestbook is a place where visitors leave a note -- like signing a guestbook at a house or library.
How it could work
The guestbook at kaimac.org is a good reference: a simple form (name, website, message, captcha) that posts to a small backend. Messages are stored and displayed in reverse-chronological order. No bloated comment system, no login, no cookies.
On cbueth.de the guestbook could work similarly: a lightweight Python CGI script (or Go binary) behind nginx that accepts POST requests, writes to a local file or SQLite database, and renders the messages server-side. A simple captcha (math question or text-based) keeps spam out without JS.
Coming soon. It will be activated once the VPS backend is ready. If you'd like to say hello before then, email me or reach me on Matrix.