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@goatsarah @jupiter_rowland @mike Prefixing this with the comment that I haven't read the ActivityPub spec nor the Mastodon source but based on my experience developing Internet standards.
Mastodon Clients are communicating with Mastodon Servers via the Mastodon API (with some nodeinfo API thrown in to know it's Mastodon probably). This will define what they see as the content of a message and how they render a message. It won't matter to them if the message started out in a richer format.