As an SSG user, I prefer the cleanest markdown as input, and putting all the formatting details into the CSS. E.g. I don't need `.abstract`, the CSS will format the first paragraph as an abstract without me asking explicitly.

OTOH I see this as a way to produce more rich self-contained documents. There's no CSS, but there's a bunch of predefined styling options. I can't help but see the early HTML in it. HTML 1 did not have colors and barely any formatting, comparable to Markdown. HTML 3 had stuff already like <center>, etc.