What is CommonMark? Definition of the markdown standard

CommonMark is an unambiguous specification of markdown syntax — the standard that defines exactly how a given piece of markdown should render, used as the baseline by GitHub, Discourse, Discord, and most modern markdown tools.

The motivation is interoperability. Early markdown had ambiguous cases — exactly how deeply lists could nest, what made something a paragraph versus a heading, what characters needed escaping — and different implementations resolved them differently. The same document might look correct in one tool and broken in another. CommonMark settled the ambiguities so a document that conforms renders identically across implementations that follow the spec.

Where the spec lands in practice

For most users, the difference is invisible — they write what looks like markdown and it renders. The spec matters when a document needs to travel: across editors, into a chat surface, into an AI coding tool, into version control. A CommonMark-conformant document survives every leg of that journey unchanged.

For LLM workflows, that interoperability is a real benefit. A feedback document exported as CommonMark renders the same way for a human reviewer reading it on a web page and for a coding agent ingesting it from a markdown file.