public struct HTMLConverter: MarkupVisitor {
    public func defaultVisit(_ markup: any Markdown.Markup) -> Node {
        .element(..., .fragment(markup.children.map { visit($0) }))
    }

    public func visitText(_ text: Markdown.Text) -> Node {
        .text(text.string)
    }

    // Override relevant `visit` implementations
    ...
}

let markdown = """
A ***basic*** document.
"""
let document = Document(parsing: markdown)
var htmlConverter = HTMLConverter()
let html = htmlConverter.visit(document)


print("""
## Original document structure:
\(document.debugDescription())

## Resulting HTML:
\(render(html))
""")