Nits:

* Introduction: "Combined with the HTTP CONNECT method (see Section 9.3.6 of [HTTP]), it allows proxying the majority of a Web Browser's HTTP traffic." This is slightly confusing to readers who aren't familiar with context; suggest: "... allows proxying HTTP/1 and HTTP/2 (which are TCP-based) and HTTP/3 (which is UDP-based).

* Throughout, 'header' should be 'header field'