mixmark-io/turndown ? reverse-engineered prompt

I need a JavaScript library for converting HTML into clean Markdown. It should work in both Node.js environments and directly in the browser.

The main feature I'm looking for is a simple function that takes an HTML string, or even a DOM element, and outputs the corresponding Markdown. I'd like to be able to customize the output style—for example, choosing whether headings use the 'atx' style with hash symbols or the 'setext' style with underlines, or picking different characters for bullet points and bold text.

It should also support common Markdown flavors like GFM, so it can handle things like tables and strikethrough text correctly. Please also build a simple demo index.html page that has a text area for HTML input and a panel that shows the converted Markdown in real-time.