Markdown dialect

OpenMarkdown reads and writes plain CommonMark with GitHub Flavored Markdown (GFM). Your files stay ordinary .md: any other editor, GitHub, or a plain text viewer can open them.

What renders

You write	You get
`# …` to `###### …`	Headings, H1 to H6
`bold` · `italic` · `~~strike~~`	bold, italic, ~~strikethrough~~
`inline code`	Inline code
```lang … ```	Fenced code block with syntax highlighting
`[text](url)`	Links
`![alt](path)`	Images, from local paths and URLs
`- item` · `1. item`	Unordered and ordered lists
`- [ ]` · `- [x]`	Task lists; the boxes stay clickable
`> quote`	Blockquotes
`---`	Horizontal rule
`\| a \| b \|`	Tables

Where it differs

Each writing below is either something OpenMarkdown accepts beyond GFM, or something GFM accepts that OpenMarkdown reads more narrowly. For each one: what it looks like on GitHub and in plain text, so you know what other readers of the file will see.

Callouts

A blockquote whose first line is exactly one of [!NOTE], [!TIP], [!IMPORTANT], [!WARNING] or [!CAUTION] (any case) renders as a coloured callout.

> [!WARNING]
> Back up before you migrate.

On GitHub: the same callout (GitHub calls them alerts).
In plain text: an ordinary blockquote with [!WARNING] on its first line.

Callouts with a fold mark

Obsidian adds - or + after the type (> [!note]-) to make a callout collapsible. OpenMarkdown recognises the mark and renders the callout like any other. It does not fold it.

On GitHub: not recognised as an alert; it shows as a plain blockquote with [!note]- visible.
In plain text: unchanged.

Math

Inline $…$ and block $$…$$ TeX render as math.

Inline $E = mc^2$, and a block:

$$
\int_0^1 x^2\,dx = \frac{1}{3}
$$

On GitHub: rendered as math too.
In plain text: the TeX source, with its $ delimiters.

Agent mentions

@agent and @agent(name) are highlighted, so a request to your agent stands out in a note.

- [ ] @agent shorten this paragraph
- [ ] @agent(reviewer) check the spelling

On GitHub: plain text in a repository file. (In issues and comments, GitHub may treat @agent as a mention of a real user.)
In plain text: unchanged.

Frontmatter

YAML between --- lines at the very top of a file renders as a Properties block.

---
title: Launch plan
tags: [launch, q4]
---

On GitHub: a key/value table.
In plain text: the YAML, with its --- lines.

Image size

A width, or a width and height, after a | in the alt text sets the image's size. Dragging an image's corner in the editor writes the same thing.

![Diagram|300](diagram.png)
![Diagram|300x200](diagram.png)

Inside a table cell, write the | as \|.

On GitHub: the image at its own size; |300 stays in the alt text, out of sight.
In plain text: unchanged.

Strikethrough takes exactly two tildes

This one is narrower than GFM. Only ~~text~~ is strikethrough. A single ~text~, and three or more tildes, stay as you typed them. This matches Obsidian and keeps things like H~2~O readable.

On GitHub: ~~text~~ is strikethrough too, but GitHub also strikes ~text~.
In plain text: unchanged.