Will a web browser ever send a request with the Transfer-Encoding header?

Question

Note

• This question is about requests with the Transfer-Encoding header, not responses
• Yes, requests can use the Transfer-Encoding header
  • Transfer-Encoding is a general-header
  • Requests may use general-headers
• One could implement a web browser that does anything, but for the purposes of this question, please limit the definition of "web browser" to Google Chrome, Mozilla Firefox, Microsoft Edge, and Apple Safari.

Background

• For XMLHttpRequest, Transfer-Encoding is a forbidden header name
• For fetch, Transfer-Encoding is a forbidden header name
• For links or forms in HTML, there is no explicit reference to Transfer-Encoding in [the HTML standard] (https://html.spec.whatwg.org/)
• It's not an authoritative source, but the PortSwigger article on request smuggling states that "Browsers do not normally use chunked encoding in requests, and it is normally seen only in server responses."
• The only reference I've been able to find that might use Transfer-Encoding is Chrome-specific, is behind an experimental flag, and requires a second, non-standard flag to enable it for HTTP/1.1

Question

• Is there a case (or cases) where a web browser will make a request with the Transfer-Encoding header?

Answer 1

I have been researching this topic recently and my understanding is that this is currently not supported by any major browser, but it is work-in-process.

The Chrome experiment is a first try to see how it will play out in web-world as it seems there are quite some edge cases that could go wrong with the complex web infrastructure there is out there (load balancers, proxies etc).

The main source I have been reading up on this is this fetch's issue about readable stream support, that I recommend to read about if you are interested.