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What Is Markdown?

Markdown is a lightweight markup language created by John Gruber in 2004. It uses plain-text syntax that converts to HTML — **bold** becomes bold, # Heading becomes an <h1>, and so on. Its goal: readable source text that renders cleanly.

Today Markdown is ubiquitous:

GitHub READMEs and PR descriptions
Documentation sites (Docusaurus, MkDocs, VitePress)
Static site generators (Astro, Hugo, Jekyll)
Note-taking apps (Obsidian, Notion, Bear)
API documentation (Swagger descriptions, Postman collections)

GitHub Flavored Markdown (GFM)

The most widely used Markdown dialect is GitHub Flavored Markdown (GFM), which extends the CommonMark spec with developer-focused features:

Tables

| Method | Endpoint       | Description        |
|--------|----------------|--------------------|
| GET    | /users         | List all users     |
| POST   | /users         | Create a user      |
| DELETE | /users/{id}    | Delete a user      |

Renders as a clean HTML table with column alignment support (:---, :---:, ---:).

Task Lists

- [x] Set up CI pipeline
- [x] Write unit tests
- [ ] Add integration tests
- [ ] Deploy to staging

GitHub renders these as interactive checkboxes in issues and PRs.

Fenced Code Blocks with Syntax Highlighting

```javascript
async function fetchUser(id) {
  const res = await fetch(`/api/users/${id}`);
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  return res.json();
}
```

The language identifier after the opening fence (javascript, python, go, bash, etc.) triggers syntax highlighting in most renderers.

Strikethrough

~~deprecated API~~ → use `/v2/users` instead

Autolinks

GFM automatically links bare URLs: https://example.com becomes a clickable link without needing [text](url) syntax.

Core Markdown Syntax Reference

Headings

# H1 — Page title
## H2 — Major section
### H3 — Subsection
#### H4 — Detail

Emphasis

**bold text**
*italic text*
***bold and italic***
`inline code`

Links and Images

[Link text](https://example.com)
[Link with title](https://example.com "Hover title")
![Alt text](image.png)
![Alt text](image.png "Image title")

Lists

- Unordered item
- Another item
  - Nested item (indent 2 spaces)

1. First ordered item
2. Second item
3. Third item

Blockquotes

> This is a blockquote.
> 
> It can span multiple paragraphs.
>> Nested blockquote

Horizontal Rules

---
***
___

Inline HTML

Markdown passes raw HTML through to the output. You can mix HTML for things Markdown can’t express:

<details>
<summary>Click to expand</summary>

Hidden content here.

</details>

Common Markdown Use Cases

README Files

A well-structured README typically includes:

# Project Name

Short description of what it does.

## Installation

```bash
npm install my-package

Usage

import { myFn } from 'my-package';
myFn({ option: true });

API

Option	Type	Default	Description
debug	boolean	false	Enable debug logs
timeout	number	5000	Request timeout ms

License

MIT


### API Documentation

Markdown is ideal for documenting REST APIs before you invest in OpenAPI:

```markdown
## POST /auth/token

Exchanges credentials for a JWT access token.

**Request body**

```json
{
  "email": "[email protected]",
  "password": "secret"
}

Response 200

{
  "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...",
  "expiresIn": 3600
}

Response 401 — Invalid credentials.


### Changelogs

The Keep a Changelog format uses Markdown:

```markdown
## [2.1.0] — 2026-03-15

### Added
- Dark mode support
- Keyboard shortcut for quick search (Cmd+K)

### Fixed
- Line ending normalization on Windows (#234)
- Memory leak in WebSocket handler (#241)

### Deprecated
- `legacyAuth()` — use `authenticate()` instead

Markdown Dialects: Which to Use

Dialect	Where	Key Extras
CommonMark	Universal baseline	Strict spec, consistent
GFM	GitHub, GitLab, Gitea	Tables, task lists, autolinks
MDX	React/Astro sites	JSX components inside Markdown
MultiMarkdown	Academic writing	Footnotes, citations
Pandoc Markdown	Document conversion	LaTeX math, citations

For most developer content, GFM is the right choice. For static sites, MDX gives you full component power.

Markdown Preview vs Local Editors

Approach	Speed	Features	No Setup
Online preview (ZeroTool)	Instant	GFM + highlighting	Yes
VS Code + extension	Fast	Full editor	Requires install
Obsidian	Full app	Wikilinks + plugins	Requires install
GitHub web editor	Online	GFM only	Requires login

An online tool is the fastest path when you’re drafting a quick README, reviewing someone’s Markdown in a PR description, or testing how a snippet will render before pasting it into documentation.

Tips for Clean Markdown

Consistent heading hierarchy — never skip levels (don’t jump from H1 to H3). Screen readers and doc generators rely on the heading tree.

Blank lines around block elements — add an empty line before and after headings, code blocks, lists, and blockquotes. Some parsers render incorrectly without them.

Prefer fenced code blocks over indented code — four-space indentation for code is ambiguous when nested in lists. Fenced blocks with a language tag are explicit and support syntax highlighting.

Reference links for repeated URLs — instead of repeating a long URL multiple times, use reference-style links:

See the [MDN documentation][mdn] for more detail.
The [MDN][mdn] site also has guides.

[mdn]: https://developer.mozilla.org

Avoid trailing spaces for line breaks — trailing two-space line breaks (line \n) are invisible and error-prone. Use <br> or restructure the paragraph instead.

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ZeroTool’s Markdown Preview renders GitHub Flavored Markdown in real time with full syntax highlighting. Paste your content, see the result instantly, and copy the rendered output if needed — no sign-up, 100% browser-based.

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