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remark is built around markdown as structured data — it parses, transforms, and serializes markdown through a pipeline of plugins. Producing HTML is not part of remark itself; instead, you bridge into the rehype ecosystem using remark-rehype, then serialize the resulting HTML syntax tree with rehype-stringify. This separation keeps each concern focused while giving you full control over every step of the transformation.

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How the pipeline works

The unified processor chains plugins together. For markdown-to-HTML, the data flows through four stages:
  1. remark-parse reads your markdown string and produces an mdast (Markdown Abstract Syntax Tree).
  2. remark-rehype converts the mdast into an hast (HTML Abstract Syntax Tree).
  3. rehype-sanitize walks the hast and removes any nodes or attributes that could be used for cross-site scripting (XSS).
  4. rehype-stringify serializes the cleaned hast into an HTML string.
1
Install the packages
2
Install all required packages with npm:
3
npm install unified remark-parse remark-rehype rehype-sanitize rehype-stringify
4
Build the pipeline
5
Create a module — for example example.js — and wire the plugins together with unified:
6
import rehypeSanitize from 'rehype-sanitize'
import rehypeStringify from 'rehype-stringify'
import remarkParse from 'remark-parse'
import remarkRehype from 'remark-rehype'
import {unified} from 'unified'

const file = await unified()
  .use(remarkParse)
  .use(remarkRehype)
  .use(rehypeSanitize)
  .use(rehypeStringify)
  .process('# Hello, Neptune!')

console.log(String(file))
7
Running this with node example.js yields:
8
<h1>Hello, Neptune!</h1>
9
Add GFM and frontmatter support
10
remark supports CommonMark by default. To handle GitHub Flavored Markdown (autolink literals, footnotes, strikethrough, tables, tasklists) and YAML frontmatter, add the corresponding plugins. Install the additional packages first:
11
npm install remark-gfm remark-frontmatter
12
Then extend the pipeline:
13
import rehypeStringify from 'rehype-stringify'
import remarkFrontmatter from 'remark-frontmatter'
import remarkGfm from 'remark-gfm'
import remarkParse from 'remark-parse'
import remarkRehype from 'remark-rehype'
import {unified} from 'unified'

const value = `---
layout: solar-system
---

# Hi ~~Mars~~Venus!
`

const file = await unified()
  .use(remarkParse)
  .use(remarkFrontmatter)
  .use(remarkGfm)
  .use(remarkRehype)
  .use(rehypeStringify)
  .process(value)

console.log(String(file))
14
This yields:
15
<h1>Hi <del>Mars</del>Venus!</h1>
16
Note that the YAML frontmatter block is parsed into the syntax tree but does not appear in the HTML output — remark-frontmatter consumes it so it is not passed through to rehype.

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Why sanitization matters

When you convert markdown to HTML, any raw HTML that appears inside the markdown source is preserved by default. This means a malicious author could embed <script> tags, dangerous javascript: URLs in links, or event handlers like onerror in image tags. All of these are vectors for cross-site scripting (XSS) attacks. rehype-sanitize solves this by traversing the hast and stripping elements and attributes that are not on its allowlist. The default schema follows GitHub’s sanitization rules, which is a sensible baseline for most applications. You can also pass a custom schema if your use case requires specific HTML elements to remain.
Always include rehype-sanitize in your pipeline when processing markdown from untrusted sources — such as user-submitted content, form inputs, or third-party data feeds. Omitting it can expose your application to XSS vulnerabilities.

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Using the remark shorthand

When both input and output are markdown, the remark package is a convenient shorthand for unified().use(remarkParse).use(remarkStringify). For markdown-to-HTML pipelines, however, you should use unified directly and compose the full chain as shown above, since the output format changes from markdown to HTML.

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Next steps

Extensions

Add GFM, math, MDX, and other syntax extensions to your pipeline.

Security

Understand XSS and DDoS risks when processing untrusted markdown.

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