Goldmark by @yuin is now the new default library used for Markdown in Hugo. It's CommonMark compliant and GitHub flavored, and both fast and flexible. Blackfriday, the old default, has served us well, but there have been formatting and portability issues that were hard to work around. The “CommonMark compliant” part is the main selling feature of Goldmark, but with that you also get attribute syntax on headers and code blocks (for code blocks you can turn on/off line numbers and highlight line ranges), strikethrough support and an improved and configurable implementation of
TableOfContents. See Markup Configuration for an overview of extensions.