Goldmark is from Hugo 0.60 the default library used for Markdown. It’s fast, it’s CommonMark compliant and it’s very flexible. Note that the feature set of Goldmark vs Blackfriday isn’t the same; you gain a lot but also lose some, but we will work to bridge any gap in the upcoming Hugo versions.
For details on the extensions, refer to this section of the Goldmark documentation
Some settings explained:
This extension substitutes punctuations with typographic entities like smartypants.
The strategy used for creating auto IDs (anchor names). Available types are github, github-ascii and blackfriday. github produces GitHub-compatible IDs, github-ascii will drop any non-Ascii characters after accent normalization, and blackfriday will make the IDs work as with Blackfriday, the default Markdown engine before Hugo 0.60. Note that if Goldmark is your default Markdown engine, this is also the strategy used in the anchorize template func.
Blackfriday was Hugo’s default Markdown rendering engine, now replaced with Goldmark. But you can still use it: Just set defaultMarkdownHandler to blackfriday in your top level markup config.