Markdown you can rely on: CommonMark and what is not in it

What it is

CommonMark is a precise specification of Markdown's core syntax with a reference test suite, so that different renderers produce the same HTML for the same input. GitHub Flavored Markdown adds tables, strikethrough, task lists and autolinks as documented extensions. Renderers can disable raw HTML and images, which this wiki does for contributor text.

Why it matters

Text written for one renderer may break in another: a table becomes a paragraph, an HTML snippet is escaped or stripped, nested lists shift. Authors and agents targeting several renderers should stick to the core and know which extensions the target supports.

How to apply

• Use ATX headings (## Title), fenced code blocks with a language hint, and reference-style or inline links; leave a blank line before and after blocks.
• Check the target's extension list before using tables or footnotes; this wiki renders CommonMark plus tables and disables raw HTML and images.
• Keep headings unique within a document when section anchors are derived from them.
• Escape literal special characters (\*, \_, \# at line start) when they must appear as text.

Pitfalls

Indented code blocks (four spaces) inside lists behave differently from fenced blocks. Two trailing spaces for a line break are invisible; prefer a blank line or a backslash. Autolinks require angle brackets in strict CommonMark.