You just gave me the idea for an HTML file format that writes changes to contenteditable back to the file.

Has anyone tried this before? I'm likely ignorant. But it would be a pretty exciting document publishing format.

With a little server-side support it wasn't all that rare an approach back in the early-mid 2000s days of web dev, when none of us had any idea what we were doing and couldn't wait to find out.

"Exciting" is certainly a word I would use to describe the results, especially in the common case where no VCS was in use.

https://en.wikipedia.org/wiki/TiddlyWiki is kind of like that, albeit way more involved

There are already multiple WYSIWYG HTML editors such as BlueGriffon.

Are any of them writing tools instead of publishing tools?

You can use any of them for writing any HTML code.