Jupyter-book depends upon MyST-NB (MyST Notebooks) which depends upon myst-parser which depends upon markdown-it-py (which is a port of markdown-it (TypeScript)) for parsing [MyST] Markdown.

https://github.com/executablebooks/myst-parser :

> MyST is a rich and extensible flavor of Markdown meant for technical documentation and publishing.

> MyST is a flavor of markdown that is designed for simplicity, flexibility, and extensibility. This repository serves as the reference implementation of MyST Markdown, as well as a collection of tools to support working with MyST in Python and Sphinx. It contains an extended CommonMark-compliant parser using markdown-it-py, as well as a Sphinx extension that allows you to write MyST Markdown in Sphinx.

https://github.com/executablebooks/markdown-it-py :

> Follows the CommonMark spec for baseline parsing; Configurable syntax: you can add new rules and even replace existing ones; Pluggable: Adds syntax extensions to extend the parser (see the plugin list), High speed (see our benchmarking tests) ; Safe by default

https://github.com/markdown-it/markdown-it :

> Follows the CommonMark spec + adds syntax extensions & sugar (URL autolinking, typographer), Configurable syntax!; You can add new rules and even replace existing ones; High speed; Safe by default; Community-written plugins and other packages on npm