"Flattening" markdown with embedded HTML into pure markdown?

[lukehutch]

I need to be able to process markdown and replace embedded HTML content with the nearest markdown equivalent, e.g. replacing an HTML img element with a markdown embedded image link. The Copy Down library is great for HTML-to-MD conversion. However, I don't know how to use the Visitor pattern in CommonMark to replace an inline or block HTML element with markdown text. Can you please provide some guidance on how to do this?