HTTP Trailers
The 1.1 version of HTTP adds chunked encoding and with it the support to send some headers after the response body.
I'd also like to see this added if anyone has the data...finding a lot of conflicting info out there and I'm looking to use it in some libraries.
Firefox supports HTTP trailers but only for Server-Timing and only over HTTPS. (Related to #3674)
Chrome supports trailers only on HTTP/2.
I think Chrome supports trailers only to a degree that it ignores them and that they do not break things. They are not available over HTTP2 at all, nor Sever-Timings over HTTP2 trailers appear in DevTools.