The passthrough extension code looks great, and my original thought was that the minor points below doesn't matter, but since:

• This passthrough sets a standard for how future extension packages should look.
• Public API is hard(er) to change after people start using it.

But note: There's no rush in doing this; Hugo is currently the only user of this, and that's an easy upgrade.

There's 2 suggested improvements:

1. Avoid repeating the package name in types and funcs

The passthrough package will never contain other things, so we might as well make it:

• NewPassthroughBlockRenderer() renderer.NodeRenderer => NewBlockRenderer() renderer.NodeRenderer
• NewPassthroughWithDelimiters => NewWithDelimiters
• PassthroughInline struct => Inline struct.
• ... others where relevant, but keeping it in identifiers such as KindPassthroughBlock

2. Add a config struct to the main New func

Over time, there will always be a need for additional configuration, so we might as well prepare for it.

func NewPassthroughWithDelimiters(
	InlineDelimiters []Delimiters,
	BlockDelimiters []Delimiters) goldmark.Extender {
}

Becomes:

func New(c Config) goldmark.Extender {
}