[_flux]

I'm going to assume many HTTP libraries and ad-hoc implementations expect one HTTP response for one HTTP request (but still support pipelining) and will break if this underlying assumption changes.

In fact, this may even be ratified in the interfaces they provide: think synchronous "give contents of this URL" functions.

[majewsky]

That's the most significant criticism of RFC 8297 (the 103 Early Hints response) from my cursory reading of the RFC. It should only be allowed when the client indicates that it can process 1xx responses. All other 1xx responses are like that AFAIK, e.g. 100 Continue is only sent when the client sets the "Expect: 100-continue" header. So a client that doesn't do that doesn't need to care about 100 Continue responses.

[nextaccountic]

The client should send Expect: 103-early-access in order for the server to send it. :(