Markdown preview is a solved problem. This exists for the two things most quick previewers leave out.

Maths and diagrams, as you type

Write $$E = mc^2$$ and KaTeX typesets it immediately. Open a fenced ```mermaid block and it becomes a rendered flowchart, sequence diagram or gantt chart in the preview pane.

Both matter for the same reason. Without them, checking whether an equation or a diagram is right means committing, pushing, and waiting for a CI render. That’s an absurd loop for a formula you’re not certain you typed correctly. GitHub renders both perfectly well; you just can’t see it until after you’ve published.

Everything else

GitHub Flavored Markdown, so tables, task lists, strikethrough and autolinks all behave. Syntax highlighting in fenced code blocks through highlight.js. Multi-file tabs, which is handy when a README and its notes want to sit side by side. Four layout modes covering editor only, preview only, split and centred, with synchronised scrolling between panes. Find and replace, a formatting toolbar, keyboard shortcuts, export to HTML or PDF, and a dark mode.

Where your work lives

localStorage, nowhere else. No account, no sync, no server, which means clearing browser data deletes it. Export anything you want to keep.

That’s the deliberate trade. There’s no server to lose your data and no server to read it either, but equally none to recover it from.

How it’s built

Vanilla JavaScript with four CDN libraries: Marked for parsing, highlight.js for code, KaTeX for maths, Mermaid for diagrams. No build step, no bundler, no framework.

Which is why it still works untouched. There’s nothing in it that rots. No dependency tree to audit, no toolchain that stops building on a new Node version. Static files on static hosting.

What it isn’t

Not a document manager. The tabs are per-session convenience rather than a library, and if you want a file workspace with revision history, this isn’t it. No collaboration or sharing, since zero backend means zero shared state.

PDF export goes through the browser’s print pipeline, so complex layouts and page breaks come out approximate. And Mermaid and KaTeX are both large, so the first load pulls them from a CDN and there’s a visible delay before diagrams appear on a slow connection.

Does the preview match GitHub?

Close, not identical. Marked with GFM covers the syntax, but GitHub applies its own stylesheet and a handful of extensions on top. For a README what you see here is reliable for structure and content. Check the final rendering on GitHub if precise styling matters.

It won’t work fully offline, incidentally, because the libraries come from a CDN. Clone the repo and vendor them locally if you need that.