Markdown syntax guide: from basics to advanced GFM

Markdown has two layers: CommonMark, the strict standard most renderers respect, and GitHub Flavored Markdown (GFM), the practical superset that adds tables, task lists, autolinks, and strikethrough.

This guide walks through both, with notes on what works where.

CommonMark — the foundation

CommonMark is what every renderer agrees on. If you stick to this subset, your document will look the same everywhere.

Paragraphs and line breaks

Paragraphs are blocks of text separated by blank lines. Inside a paragraph, a line break needs two trailing spaces or a single \ at the end of the line.

Inline elements

**strong**, _emphasis_, `code`, and [a link](https://trymarkdownviewer.com).

Block elements

• # Heading (1–6 levels)
• > Blockquote
• - Bullet and 1. Ordered
• fenced code blocks
• --- horizontal rule

GitHub Flavored Markdown extensions

GFM adds the features you actually want for technical writing.

Tables

| Feature        | Status |
| -------------- | :----: |
| Live preview   |   ✅   |
| Autosave       |   ✅   |
| PDF export     |   ✅   |

Task lists

- [x] Write the post
- [ ] Publish it

Strikethrough

~~Old plan~~ → new plan.

Autolinks

URLs and emails get linkified automatically: https://trymarkdownviewer.com.

Code fences with language hints

```ts
const message = "Hello, Markdown!";
```

The language hint enables syntax highlighting in our preview (and on GitHub, GitLab, and most documentation sites).

Frontmatter

Static site generators read YAML-style frontmatter at the top of a file:

---
title: "My post"
date: "2025-10-21"
tags: ["markdown", "syntax"]
---

It's not part of CommonMark, but it's how git-friendly content stays portable.

Footnotes (GFM)

Here's a footnote.[^1]

[^1]: And here's the explanation.

Admonitions and callouts

These vary by renderer. GitHub uses:

> [!NOTE]
> Highlights useful information.

> [!WARNING]
> Critical information that could go wrong.

Markdown Viewer renders these as styled blockquotes.

What to avoid

• HTML inside Markdown — most renderers sanitize it for security.
• Mixing tabs and spaces for indentation — pick one and stick to it.
• Trailing whitespace anywhere except intentional line breaks.

The Markdown Viewer formatter cleans up these issues automatically. Hit Format markdown in the sidebar to apply our conventions in one pass.

Where to go next

• What is Markdown?
• The Markdown cheat sheet
• The best Markdown editors