CommonMark

The 2014 standardization of Markdown’s core syntax (John MacFarlane, Jeff Atwood et al.), maintained at spec.commonmark.org. Born from the original Markdown’s deliberately unspecified edges — a decade of “whose nested list is correct?” resolved by a rigorous, versioned, testable specification with a reference implementation (cmark) and an exhaustive conformance suite.

What it standardizes: paragraphs, ATX/setext headings, emphasis (the asterisk/underscore rules), lists (the indentation rules), code blocks/spans, blockquotes, links/images, HTML blocks, hard breaks. What it deliberately does not include: tables, task lists, strikethrough, autolinks — the core stays minimal so dialects can layer extensions without contradicting the base (“a strict base, not a ceiling”).

Position in the family: the base layer of the ladder — GitHub Flavored Markdown is a strict superset (“all CommonMark documents are valid GFM”), and Obsidian Flavored Markdown builds on the same base. Content that stays inside CommonMark renders correctly in essentially every Markdown tool made since ~2016; it is the portability floor for anything that travels between vault, repo, and chat.

Reference implementations: cmark (C, the reference), commonmark.net, markdown-it (what the web runs on), plus ports in Rust (comrak — also the GFM engine), Go, Python (markdown-it-py).

Related: Markdown Flavors · GitHub Flavored Markdown