@JorgeStolfi @cstross @jupiter_rowland I think that multi-part content-types as embedded protocols rather than built-ins is more the consequence of a technical implementation choice (in the standards) than something fundamentally different in a logical sense.
The normative spec also explicitly acknowledges the lack of an expectation for full support of everything other instances might support (https://www.w3.org/TR/activitystreams-vocabulary/#introduction).
@JorgeStolfi @cstross @jupiter_rowland In practical terms when you get something with no available fallback what happens is that you simply get a link by your instance (if you're using #Mastodon) to view the Activity on its originating instance which presumably has the means to display it.
If you have a thin-server that uses a fat client, that assumption will be violated however.
You can currently see that when following #Peertube accounts from Mastodon.