> HTML is a subset of Markdown.

Yes, so all websites are already written in markdown. Its just tjat No browsers support the typical header/list shorthands, but that doesn’t matter because even if they did you’d still need more tages to get interactivity and styling working.

People aren’t imagining a world where markdown makes anything simpler they are imagino by that a format change would make people build less complex websites. But why would they? It’s not HTML that makes the Twitter front end or Facebook complicated, it’s the desisted functionality, which wouldn’t change even if the spice code looked more like markdown and less like html.