Free MD Viewer

A markdown viewer and editor that runs entirely in your browser. Your files stay on your machine — there is no upload endpoint, no account, and no server-side rendering.

Live at kingsbridge-consultancy.com/md-viewer

Drop a .md file on the page (or click Open), read it properly rendered, edit it with a live preview, and save it straight back to disk. The whole app is one self-contained index.html (~3.6 MB, ~1 MB over the wire) with every library and font embedded, so you can also save the page and run it offline from file://.

Why

Markdown files are everywhere now — every AI tool hands you one — but double-clicking a .md file still gets you a wall of raw text, and most online viewers upload your document to a server to render it. For work documents and private notes, that's a real cost of a "free" tool. This renders locally instead.

The claim is checkable rather than promised: everything is inlined, so open devtools and watch the network tab, or read the source with view-source. Only the vendor libraries are minified — the app's own code ships readable.

Features

• Drag & drop or file-picker open; accepts .md, .markdown, .txt
• Live split preview (Edit / Split / Preview modes) with scroll sync; phones open in Preview
• Save writes back to the original file via the File System Access API (Chrome/Edge — works for both picked and dropped files); Save-As and a plain download fallback everywhere else
• GitHub-flavored markdown: tables, task lists, strikethrough, fenced code blocks
• Syntax highlighting in code fences (highlight.js, GitHub-style palette for both themes)
• Mermaid diagrams from ```mermaid fences, theme-aware (re-rendered on dark/light toggle)
• LaTeX math via KaTeX: $inline$, $$display$$, \(...\), \[...\] — math is extracted before markdown parsing so underscores/asterisks in TeX survive, and $ inside code spans and fences is left alone
• GitHub callouts: > [!NOTE], [!TIP], [!IMPORTANT], [!WARNING], [!CAUTION]
• Export (all generated in-page, nothing uploaded, no watermarks or limits):
  • .html — the rendered document as one self-contained file, styles and KaTeX fonts embedded
  • .png — the preview rasterized via an SVG <foreignObject> painted onto a canvas, at up to 2× device scale. Remote images can't load inside an SVG-as-image, so they come out blank; data: URIs are fine
  • PDF — the browser's own print-to-PDF, so the text stays selectable. Printing forces the light palette (and temporarily switches the app to light first, since Mermaid bakes theme colors into its SVG)
• Per-block downloads — hover a Mermaid diagram for an SVG button (real dimensions taken from its viewBox, not mermaid's width="100%"), or a table for a CSV button (RFC-style quoting, UTF-8 BOM so Excel reads accents correctly)
• Installable (Chrome/Edge desktop) — a service worker plus a manifest declaring file_handlers, so once installed, double-clicking a .md file opens it here. The launch handle is a real FileSystemFileHandle, so Save writes straight back to the file you opened. Works offline
• Table of contents — toggleable panel built from headings; a slide-over drawer on phones
• Reader controls — text size and line width (Aa button), persisted
• Sanitized rendering — untrusted markdown can't inject script (DOMPurify, including over KaTeX output)
• Draft auto-restore — content persists in localStorage, so a refresh or crash loses nothing
• Dark / light theme (follows system, manual toggle remembered)
• Inline rename, word/char/line counts, ⌘S / ⌘⇧S / ⌘O shortcuts

Browser support

Rendering, editing, exports and per-block downloads work in any current browser. Two features depend on Chromium-desktop-only APIs and degrade gracefully elsewhere:

Building

No package manager, no toolchain — just Python 3 and bash:

./build.sh

That inlines the vendored libraries and the KaTeX fonts into src/index.template.html and writes index.html, then copies the PWA sidecars to the repo root. Edit src/index.template.html, never index.html.

src/index.template.html   the app (HTML/CSS/JS), with markers where libraries get inlined
vendor/                   the bundled libraries + KaTeX .woff2 fonts
pwa/                      manifest.json, sw.js and the icons
tools/make-icons.py       regenerates the icons (hand-rolled PNG encoder, no dependencies)
build.sh                  the build
index.html                the built, self-contained app (committed)
licenses/                 full license texts for everything bundled

Self-hosting

index.html is the entire product. Copy it anywhere a static file can be served, or open it from disk. It needs no CORS, no CSP beyond allowing its own inline scripts, and no backend.

The manifest.json, sw.js and icon-*.png files beside it are only needed for the installable-app behavior; serving index.html alone works fine without them. Installability additionally requires HTTPS (or localhost).

License

MIT for this project's own code.

The built index.html bundles marked, DOMPurify, highlight.js, KaTeX and Mermaid, so their licenses travel with any copy you distribute — see THIRD-PARTY-NOTICES.md and licenses/. A summary is also embedded as a comment at the top of the built file.

Contributing

Issues and pull requests are welcome. Two things to keep in mind:

1. Edit src/index.template.html and run ./build.sh; don't hand-edit index.html.
2. Anything that would send a user's document off their machine — telemetry, cloud rendering, AI features backed by an API, share links that upload — is out of scope by design. Features that need a server belong in a fork, not here.