No, and technically they cannot be.
The editor is a pure front-end HTML file with no backend. Opening, editing, previewing and saving all happen in your browser; saving goes through the browser's File System Access API straight back to your own disk.
If you want to confirm it yourself: open your browser's developer tools → Network tab → open a folder and save. You will see no request carrying your file contents anywhere.
Because writing back to the original requires the File System Access API, which so far only Chromium-based browsers implement.
Firefox and Safari can edit and preview normally, but saving falls back to downloading a new copy — they cannot overwrite the file you opened. The details are in browsers and file access.
The File System Access API only exists in a secure context. Over plain http:// the page
still loads, but "open a folder" and "save in place" disappear silently — no error message,
the capability is simply absent.
fish-zero.com forces HTTPS everywhere, so you will not hit this here. If you self-host, use
HTTPS.
You can type, preview and switch themes, but mobile browsers cannot overwrite files on disk, and the full-width three-column layout is awkward on a small screen. This tool is built for the desktop.
Yes, if you save the file locally. Since 2026-08-12 the Markdown parser and the monospace font are embedded in the HTML file itself, so the editor makes zero external requests — open it from your disk or a USB stick, pull the network cable, and everything still works.
⚠️ Opening fish-zero.com/MDEditor while offline does not work yet. That needs a service
worker to cache the page, which is not implemented. To work offline, save the file first.
(Body text uses fonts already on your computer rather than downloaded ones, so it is unaffected either way.)
Yes. The editor is a single index.html released under the MIT license — keep it on a USB
stick, an intranet, or your own server.
⚠️ If you self-host, remember to use HTTPS (see above). The downloadable version never contains ads or tracking code, and an automated check in the build enforces that.
Only two things stay in your browser, and both can be cleared at any time:
| Where | What |
|---|---|
localStorage | Theme, interface language, and whether the sidebar and editor pane are open |
IndexedDB | A reference to folders you authorised (not file contents), so a refresh does not lose your place |
Clear them through your browser's site-data settings. You can also revoke file system access in the site permissions.
That is the folder handle in IndexedDB mentioned above. It is an opaque reference the
browser gives out — it contains no file content, and the browser may still ask you to
re-authorise.
The preview renders with marked.js, covering the usual CommonMark syntax. On top of that, YAML front matter is detected and rendered as its own card instead of being mangled — see YAML front matter.
Common syntax is collected in the syntax reference.
Yes — that is dual preview: the left pane follows the file you are editing, the right pane
holds any other .md for reference, with optional synchronised scrolling. See
dual preview.
No, deliberately. The editor overwrites the original file on your disk, so autosave would mean "the change you were still unsure about has already been written" — and there is no undo.
The red dot next to the filename means there are unsaved changes; nothing is written until you
press Ctrl/Cmd + S.
Email [email protected], or see Contact. ⚠️ Before attaching a screenshot, check that it does not show your own document.