TiddlyWiki is a standalone single HTML file wiki that can save itself by offering a download of modified copy of itself or storing itself on one of the many available hosting providers, e.g. saving itself to GitHub, GitLab, WebDAV etc. All these options can be removed to save space and another option to use sendToChat() API added instead.
It is a bit too large by default, empty edition is 2.4M, but for an experiment this restriction can be removed or maybe it is possible to reduce it enough by dropping unnecessary default plugins and saving methods. Also gzipped the file empty.html.gz is only 427K, so inside an .xdc ZIP archive it can be small.
Then the user workflow would be to save a prepared empty version of TiddlyWiki.xdc to Saved Messages, open it, edit and save a new copy via “send to chat”. No need to send any webxdc updates or merge changes, as TiddlyWiki is usually used as a personal wiki. Sending it to some chat would also be possible, but if two users edit at the same time they will send two different updated copies to the chat then and have to merge them manually.
Dokuwiki is written in PHP and requires a server to run. DokuWiki is a personal wiki that anyone with a web browser can run locally to store their notes without the need to have a PHP-supporting server somewhere.
It is a single 700kb file, compressed into a zip file takes just a little bit more than 200kb and every time you make a change the file is updated and re-downloaded with the changes.
Of course it would be nicer to be able to edit it simultaneously.