mathdown

Online collaborative MarkDown editor. Single-pane (Zettlr like) in-place formatting, punctuation visible. Renders $LaTeX$ math formulas in-place. Collaboration is Etherpad-like: no accounts, each doc has sharable secret URL. Con: must bookmark, no permissions, no doc deletion! (Frontend is open source, but backend is Firebase.)

• Serious scientific users should also try Overleaf's rich text mode and/or \usepackage[hybrid]{markdown}