I looked for threads about it on various mailing lists and wasn't able to track anything down. If you have links, I'd love to take a look at the past discussion.
I actually really like the idea of doing it in the Shadow DOM.
That's a great point, and definitely an obstacle, but if there's interest I don't think it should stop us from at least making the attempt. In fact, I think it's encouraging that Jeff Atwood is interested in standardizing Markdown, because it might mean he, his readers, and other like-minded folk might be interested in getting involved if we try to revive the effort.
Some flavours of Markdown actually do allow for adding classes and IDs. See, for example, Markdown Extra’s special attributes. That said, I agree that no matter what flavour of Markdown gets encoded, it's not going to be a perfect match for HTML and its semantic or syntactic power. I don't think it needs to be, though.
The use case isn't to use Markdown for the entire web, it's to address a specific pain point where Markdown is already being used: user-generated content. Most websites that use Markdown now do so for comments, posts, and other content generated by users that either don't need a lot of detailed control, don't have the technical knowledge to write full HTML, or where full HTML can't be trusted.
It could happen, but I'd argue that if there's enough interest in actually standardizing it, implementing it, and maintaining it, then that isn't necessarily a bad thing. This possibility is why I'd prefer the
<pre formatting="markdown"> approach, for better extensibility later.
That does make sense, but then we'd be stuck with
<markdown> instead of, as I mentioned above
<pre formatting="markdown">. Not the end of the world, but it's not as clean IMO. Also, I'm not sure what benefit a custom element would have over the existing server- and client-side solutions out there. In fact, we can already see that many sites are using Markdown, even if it's not “every site on the internet”.
In general, I agree that's probably the right approach. But seeing as how we already know that Markdown is widely-used, and that an actual element is probably not the best solution, I don't think a custom element is the right approach here.