How to convert HTML to Markdown while retaining non-markdown HTML tags?

I'd like to be able to take an existing HTML snippet and convert it to markdown. I've tried pandoc for this purpose: ``` pandoc test.html -o test.md ``` where test.html looked like this: ``` Hello <!-- more --> and some more text <h2>some heading</h2> ``` The result was this: ``` Hello and some more text some heading ------------ ``` Thus, it not only converts tags that have a direct meaning in markdown. It also removes tags that I would like to retain as HTML (e.g., HTML comments, `iframe` tags, and so on). - How can I convert HTML to markdown in a way that any tags that don't have an equivalent in markdown are retained as raw HTML? - More generally how can I have control over how the HTML to markdown conversion is done? In particular, I'd be interested in command-line program options. For example, perhaps there are options that can be supplied to pandoc.