ActivityPub assumes content is text/html by default when mediaType isn't specified, whether the object is a Note or an Article. Misskey does something different, for reasons I still don't fully understand. It stores its internal representation as MFM, its own markup language. Between Misskey instances, it sends that through a separate _misskey_content property rather than content itself. When talking to non-Misskey software, it converts HTML into MFM.

The trouble is that MFM can represent much less than HTML. If another implementation sends HTML that has no clean MFM equivalent, the formatting comes out mangled in Misskey.

I keep wondering why Misskey didn't just store HTML, like Mastodon and most other implementations do, or at least kept both the source markup and the rendered HTML side by side. I know storing only HTML can lose some MFM features too. I still don't understand why Misskey chose the format that can represent less.