Quill

Quill is a Progressive Web App. Install it from your browser's install button (the icon in the address bar, or menu → “Install Quill”). Once installed you can send Markdown to it from your operating system:

• Double-click a .md / .markdown file, or use Open with → Quill, and it opens right here.
• Share text or a link to Quill (where the OS supports web share targets) and it loads into the editor.
• Open / Save / Save As edit local files in place. Save writes straight back to the file you opened.

File System Access (in-place save) and OS file-handlers are Chromium-only (Chrome, Edge, Brave, Arc…). In Firefox, Safari, and other browsers Quill automatically falls back to classic upload to open and download to save. Everything still works, files just aren't edited in place.