Ready with minor nits.

In 3., two documents about "extended CONNECT" are mentioned. Both are about WebSockets. Would it make sense sayint it here, potentially sparing readers the indirection?

4.3: it's (to me) not complely clear what is recommended. Set limits on bytes or time? The subsequent sentence indicates that this would be a problem as well.

IANA: maybe link to the HTTP Proxy Error Types registry - that would make it easier to check the requirements.