7.5 briefly touches this topic.

Responses (both final and interim) can be sent at any time after a request is received, even if it is not yet complete

More explicit descriptions are helpful, such as:

1. responses can be sent at any time, successful for failed ones
2. responses may be completed before requests are completed. This is described for100-continue but should not be limited to 100-continue.
3. for 2), should a client that sees a failure response continues sending the remaining request content?