OK, I think this is a bug. However, if it turns out that it actually is not a bug, then I look forward learning why it isn't.
While the option of using HTML to format your entire post is possible with DIscourse, there seems to be a problem quoting from a post which used HTML as the markup language.
What appears to happen is that all of the `<` & `>` characters around the HTML tags are converted to the `<` and `>` symbols. So, `
` in the original HTML becomes `<p>` when the post is quoted. The result is that the quoted text is unformatted and IMO unreadable.
I'll provide an example of this in a reply to this post below …
How could this ever be a problem worth fixing? Who would be so insane as to create not just one, but thousands of Discourse posts using only HTML? It happened to the tudiabetes.org site. When the site was migrated from Ning to Discourse the Ning posts were copied/migrated to the Discourse site "as is" (I am guessing). This "bug" makes it infeasible to quote anything for reference from the migrated HTML posts.