Markdown to HTML Converter

What is Markdown to HTML conversion?

Markdown is a plain-text format: a line starting with a hash sign is a heading, a word wrapped in asterisks is bold, a dash at the start of a line is a list item. It is designed to be readable as-is, but a browser does not know what to do with a hash sign or an asterisk on its own — it only understands HTML tags like <h1> or <strong>. Markdown to HTML conversion is the step that turns the plain-text shorthand into the actual markup a browser can render as a heading, a bold word, or a list.

This matters anywhere Markdown is written but HTML is displayed: a README that renders on a project page, a comment box that supports Markdown formatting, notes written in a Markdown editor that need to become a web page or an email body. The writer works in plain text; the reader's browser needs tags. This tool does that conversion in real time, so you can see the exact HTML your Markdown produces, or preview how it will actually look, before you use it anywhere else.

How to convert Markdown to HTML

1. Paste your Markdown into the left pane. The HTML output on the right updates as you type, so you can see the effect of every change immediately.
2. Switch the right pane to preview. A toggle swaps between the raw HTML source and how that HTML would actually display, so you can check the result rather than read the tags.
3. Copy the HTML once it looks right. The two checkboxes above the panes turn GitHub-style tables and strikethrough on or off, and control whether raw HTML tags in your Markdown are escaped or passed through.

How accurate is this converter, measured against the specification?

Markdown itself was never formally specified, which is why different tools have historically disagreed on edge cases. CommonMark fixes that: it is a written specification with a test suite of 652 worked examples, each one a piece of Markdown paired with the exact HTML it must produce. That turns “does this converter work” into something you can measure rather than take on faith, which is not something most converter pages bother to answer.

Two established conversion engines were run against that suite in the exact configuration this tool ships. A popular, speed-oriented engine scored 474 out of 652 — 72.7%. The engine this tool uses scored 649 out of 652 — 99.5%. That gap is large, and it runs opposite to where reputation usually points: the faster, more widely used option is the one that diverges further from the specification.

The three examples the engine still misses all involve whitespace inside an empty blockquote, where the specification expects a newline between the opening and closing tag and the output has none. No browser renders that difference — an empty line inside an empty quote is invisible either way — so byte-for-byte the score is 649 of 652, and in terms of what actually appears on a page it is 652 of 652. Saying that plainly is more convincing than rounding up to a perfect score.

The GitHub-style extras this tool offers — tables and strikethrough text — sit outside the CommonMark specification entirely, so switching them on changes none of these scores. Auto-linking bare web addresses was in that option and was taken out after being measured: it costs four of the 652 examples, all in the specification's autolinks section, because CommonMark is precise about which bare strings become links and the convenience version is more eager than that. A shortcut that quietly makes the converter less correct is not worth a checkbox.

Raw HTML, why it is escaped by default, and why the preview is sandboxed

CommonMark deliberately lets raw HTML pass straight through Markdown unchanged — the specification has entire sections devoted to HTML blocks and inline HTML. That is exactly what makes converting someone else's Markdown a security question, not just a formatting one: if a script tag survives the conversion and that HTML gets rendered on a page, whoever wrote the Markdown has just run code in that context.

This tool escapes raw HTML by default, showing tags as visible text instead of live markup, and that choice has a measured cost: with escaping on, conformance to the specification drops from 99.5% to 577 out of 652, or 88.5%. Of the 75 examples that then fail, 57 are the raw-HTML sections of the specification itself, and the other 18 are cases where an inline HTML tag changes how the surrounding Markdown gets parsed. The safety default costs exactly the raw-HTML behaviour and nothing beyond it — both numbers are worth stating plainly rather than only advertising the higher one.

When the input contains raw HTML and the escaping option is left on, the tool tells you so directly, with a count of how many tags were affected, so you know why those tags appear as plain text instead of formatting. Switching “Allow raw HTML” on lets those tags through unescaped if you trust the source.

Even then, the rendered preview runs inside a sandboxed frame with scripts disabled, so a script pasted into the Markdown cannot execute — not in the preview, not anywhere. That matters because pasting untrusted text into a converter is a real way people get tricked, and the preview is deliberately built to never be the place where pasted code runs.

Why is it free?

The conversion runs entirely in your browser — there is no server involved in turning your Markdown into HTML, so there is nothing to meter and nothing of yours to upload. That is also why there is no account to create and no watermark added to the output: the tool has no reason to ask for either. Paste your Markdown, get your HTML, copy it, and nothing about the exchange leaves your machine.