Most of these wouldn’t require a div. They could be marked up with either blockquotes or line blocks (pre), plus additional markup for the extra data. Here’s what I suggest:
- Collapsable spoiler, with title. Use the blockquote syntax with a class for spoiler. Set a parser rule to make a heading and/or link directly before the blockquote the collapsable title.
- Spoiler with hidden visibility. Blockquote with spoiler CSS class.
- Sidenotes, epigraphs. The proposed line block syntax (
|) with a specific class.
- Lyrics in text. Line blocks again.
- Forum quotes (source post & user info). Perhaps a blockquote/definition list combination, I’m not sure. Discourse solves this very specific requirement by using a custom tag instead of Markdown. Blockquote seems like the right element to use in any case.
The problem with using the div tag is that is contains no semantic meaning other than being a container. The HTML5 spec even advises against using it (emphasis added):
Authors are strongly encouraged to view the div element as an element of last resort, for when no other element is suitable. Use of more appropriate elements instead of the div element leads to better accessibility for readers and easier maintainability for authors.
This is a strong indicator that the div element should not have it’s own Markdown syntax, even as an extension. We should aim to add more semantic elements on a case by case basis instead.