I built an online viewer and editor for .md files where the file stays on your machine. Free to use and code is available under MIT License here: https://github.com/hattray/markdown-editor

You can also install it as a Chrome/Edge app so that it works for files on your desktop by double-clicking.

Supports standard .md files with formatting, syntax-highlighting for code, table of contents, Mermaid diagrams, LaTeX via KaTex. Also supports exporting to .html, .pdf and .png.

Please give it a try and share your feedback for improvements.