Markdown Preview

This renderer covers the common core of Markdown, and these things are not in it

It is a small hand-written renderer rather than a full CommonMark implementation, so it is worth knowing exactly where the edges are before you trust a preview. Everything in this list was measured against the live tool:

• Tables do not render. Each row comes out as a paragraph of pipes and dashes.
• Images do not render, and leave a stray exclamation mark. ![alt](url) becomes a plain link with a ! in front of it, because the link pattern matches and the image marker does not.
• Underscore emphasis does not work. _em_ and __strong__ render as literal underscores. Only the asterisk forms, *em* and **strong**, are recognised.
• Nested lists are not nested. An indented sub-item ends the list and becomes a paragraph with its leading spaces showing.
• Strikethrough does not work. ~~gone~~ renders literally.
• Task list boxes do not render. - [ ] todo becomes a list item whose text starts with [ ].
• Only http and https links render. A mailto: link and a relative link such as [rel](/about) are both left as literal text.

What does work: headings one to six, paragraphs, unordered and ordered lists at a single level, blockquotes, horizontal rules, fenced code blocks, inline code, asterisk bold and italic, and http or https links. For most READMEs and notes that is the whole of what gets used.

Emphasis is applied inside inline code, which it should not be

Measured: `*not italic*` renders as code containing italic text. The inline-code pattern runs first and wraps the content in a code tag, and then the italic pattern runs over the result and finds the asterisks still sitting inside it.

So a code span is not a safe place to put Markdown characters here. If you are documenting Markdown itself, or writing about a shell glob, a multiplication expression or a C pointer, the asterisks will be eaten and the surrounding text italicised. The same applies to double asterisks inside a code span.

A fenced code block, the kind opened and closed with three backticks, does not have this problem: its contents are escaped as a unit and no inline rule runs over them. If what you are writing contains Markdown characters, use a fence rather than a span.

Raw HTML in your Markdown is escaped, never rendered

Real Markdown allows HTML to pass through. This renderer escapes it instead. Measured, with the browser console watched: <img src=x onerror=alert(1)> renders as that visible text and nothing executes.

That is a deliberate trade and it cuts both ways. It means pasting a document from an untrusted source into this preview cannot run anything, which is the right default for a tool on a public page. It also means a README that uses a few HTML tags for a centred logo or a details block will show those tags as text rather than applying them, so the preview will not match what GitHub shows you.

The preview is not what your platform will show, and the differences are not small

Every Markdown renderer is a dialect. CommonMark is the specification that most of them aim at, and GitHub, GitLab and the rest each add extensions on top: tables, strikethrough, task lists and autolinks are GitHub Flavored Markdown rather than core Markdown, and all four are in the list above that this tool does not do.

So treat this as a check on your structure, headings, lists, paragraphs and code blocks, rather than as a proof of final appearance. If a table or a task list matters, preview it where it will be published.

What this tool deliberately does not do

• No tables, images, strikethrough, task lists, nested lists, footnotes, definition lists or reference-style links.
• No underscore emphasis, and no setext headings, the kind underlined with equals signs or dashes.
• No raw HTML pass-through, by choice, as above.
• No syntax highlighting inside fenced code blocks. The language tag after the fence is accepted and ignored.
• No export. There is no download and no copy-as-HTML; the preview is for looking at.
• No files, no saving. Refreshing loses what you typed.

Nothing is uploaded. The rendering happens in your browser and the text never leaves your device, which matters if you are previewing a draft that is not public yet.

Sources

• The specification most renderers aim at, including what belongs to core Markdown rather than to an extension: CommonMark. spec.commonmark.org
• Tables, strikethrough, task lists and autolinks as extensions rather than core: GitHub Flavored Markdown specification. github.github.com/gfm
• The escaping applied to your text before any rule runs, which is why raw HTML is shown rather than executed: OWASP Cross Site Scripting Prevention Cheat Sheet. cheatsheetseries.owasp.org
• Why a pattern applied after another one can match inside its output: regular expression character classes. developer.mozilla.org

Every behaviour on this page was measured against this tool as it is published, in a browser, on 14 October 2026. Where the page says what the tool does not do, that is a statement about this tool, not about Markdown in general.