I've been on this site for a short time, and I've been happy to submit plain text with only italics and boldface markup (i.e., the underscores and asterisks).
Recently I wanted to have a few single-line breaks but just a single paragraph break doesn't show up in the format previewer. I did experiment with a {br} tag and that did work in the previewer. However, there is a warning on the previewer page (the software seems to be nine years old!) that a lot of HTML may not appear on the site itself.
I've experimented with an HTLM page, but I can't get one to work in the previewer (all the tags appear as written). In any case, it's not clear from the formatting guide that the {br} tag is supported.
I'm not trying to do anything that complicated, and it seems that other sites have more modern software that would easily handle this.