I was using it but I stopped for some reason, I might give it another go. I think it had a few annoying bugs so I stopped using it.
Come to think about it, I don't think it works as an "editor" in HTML text fields?
There is also Firenvim which embeds real neovim instances inside text boxes.
There was "itsalltext"[1] (sadly defunct) - but there's an alternative (i just discovered - so I've yet to try it) : ghosttext https://github.com/fregante/GhostText
[1] https://github.com/docwhat/itsalltext
Ed: wasavi does enhance the text-area widget (wasavi is a browser extension) - so it does turn text fields into a vi-like editor. I can't imagine it plays well with vimperator-style extensions, though.