var json = jsonEncode(_controller.document.toDelta().toJson());
Do we have a Dart plugin for converting FlutterQuill to HTML content?
Recently there was an issue in their GitHub account and someone found a workaround via Markdown.
But keep in mind that it is not entirely correct yet and as far as I know there isn't any straightforward conversion into HTML yet.
Another thing you can try is to create a fork of the zefyr HTML editor, use their Delta converter, and update it to work with Quill deltas. I tried that for myself. It worked in simple cases, but in our project there were harder cases, so we ditched this solution.