Elfeed-offline currently has a Dream web server which acts as a proxy server in front of Elfeed’s Emacs simple-httpd server.

simple-httpd supports HTTP/1.1 protocol, while Dream provides transparent upgrading of connections to HTTP/2 — if the client can handle HTTP/2 and the connection is using HTTPS, it is transparently upgraded to HTTP/2.

My proxying code was too simplistic in forwarding the headers too along with the content received from the simple-httpd server. Some of the HTTP/1 headers are no longer valid in HTTP/2. And, Safari (and curl) strictly adhere to the protocol and fail if there are invalid headers. Curl, for instance, fails with the following error:

< HTTP/2 200
< server: simple-httpd (Emacs 30.1)
< date: Fri, 16 Jan 2026 10:57:25 GMT
* Invalid HTTP header field was received: frame type: 1, stream: 1, name: [connection], value: [keep-alive]
* [HTTP2] [1] received invalid frame: FRAME[HEADERS, len=77, hend=1, eos=0], error -531: Invalid HTTP header field was received
* HTTP/2 stream 1 was not closed cleanly: unknown (err 4294966765)
* Connection #0 to host 192.168.1.5 left intact
curl: (92) Invalid HTTP header field was received: frame type: 1, stream: 1, name: [connection], value: [keep-alive]

This was causing issues for @Feyorsh who was trying out elfeed-offline with Safari. Thanks for taking the time to debug the problem and for suggesting a fix!