I doubt Markdown, BBCode or anything similar is a good idea here. That's just introducing extra complexity - and what for? The point of markdown is that its simple for humans to read and write directly, which isn't applicable here.
The downside to markdown should be obvious:
* more code, both server and client-side (to implement the to-and-from conversion)
* more bugs (due to more code and the complexity of escaping valid input that happens to be markup in one or the other)
* less features (if the editor supports some html that doesn't map 1-to-1 to markdown, you're in trouble)
* less future-proof/platform independent (html isn't going anywhere, but that markdown variant you're using with the custom extensions you needed might be subtly different in whatever language/platform/toolkit you'd prefer in 5 years).
Html is by far the better choice. If there's an improvement to be had here, it's in using the (compatible) XHTML5 serialization to ease parsing. And it's quite likely already using that, since that's what browsers' rich-text-editing generally produces.
I suppose it's strange to say this on HN where the markup is, well, atrocious, but after using Markdown for ages in varieties of places, it's simply more pleasant to use for the "advanced" user who doesn't want to memorize hotkeys or highlight and press buttons to give their text some basic simple formatting.
It's a hit on reddit, GitHub and more for a good reason. They could have whitelisted things as well, but they chose not to.
The downside to markdown should be obvious:
* more code, both server and client-side (to implement the to-and-from conversion)
* more bugs (due to more code and the complexity of escaping valid input that happens to be markup in one or the other)
* less features (if the editor supports some html that doesn't map 1-to-1 to markdown, you're in trouble)
* less future-proof/platform independent (html isn't going anywhere, but that markdown variant you're using with the custom extensions you needed might be subtly different in whatever language/platform/toolkit you'd prefer in 5 years).
Html is by far the better choice. If there's an improvement to be had here, it's in using the (compatible) XHTML5 serialization to ease parsing. And it's quite likely already using that, since that's what browsers' rich-text-editing generally produces.