And by that I mean,
An http server that serves word-like rich formatted text documents and includes a web-based rich formatting text editor to write and edit these documents.
With some searching I found these might be candidates
Ghost
WriteFreely / Write.as
Plume
And maybe the following editors in some kind of standalone mode
TipTap
Editor.js
Quill.js
CKEditor 5
Slate.js
And they’ve answered their own question by listing several valid candidates.
Well, I’m hoping someone has a better one, or at least can confirm that one of the things I listed isn’t actually a huge waste of time.