Safari and invalid HTTP/2 headers

Fri, 16 Jan 2026 21:37:00 +0530

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 Some of the HTTP/1 headers are no longer valid in HTTP/2.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!

An offline-friendly Elfeed web UI

Wed, 07 Jan 2026 02:31:00 +0530

I want to read articles from my RSS subscriptions on my phone without signing up to a hosted service or running a public server.

Elfeed, the Emacs feed reader, already lets me manage my subscriptions and read from within Emacs. It comes with an experimental web UI, but needs the server (running on my laptop) to be accessible whenever I want to read. My phone becomes useless the moment my laptop sleeps.

So, I built an alternate web UI with a service worker that caches content on the client side for a smooth offline reading experience. It’s written in OCaml and compiled to JavaScript using js_of_ocaml.

Why bother with RSS feeds?

It is annoyingly easy to fall into the trap of short form videos or other algorithmically “curated” feeds and get sucked into scrolling mindlessly. I fear this is going to get even worse with all the generative AI stuff.

I want to be more deliberate about what I consume. RSS feels calmer and gives me a greater sense of control. Or it may just be nostalgia from good old Google Reader days.

Why Emacs and Elfeed?

Since Google Reader was killed, I’ve bounced between many readers but none of them has really stuck with me. I’ve had different phases of using Elfeed, over the years, though.

I like the fact that everything is local with Elfeed – no hosted services, no public servers. And since it lives inside Emacs, everything from tagging to how entries are displayed can be tweaked.

Why Elfeed offline?

Elfeed also ships a basic web UI that lets me read my feeds from a different device, but only when my laptop is online and reachable. I’d like to be able to read these posts even when I’m “on the road”, say, while waiting for a train or while taking a cab ride.

I knew it should be possible to do this with some client side caching. And, I considered improving the web UI of Elfeed itself but it doesn’t seem to be actively maintained in the last couple of years. Creating a separate project would also give me more freedom to experiment.

How does it work?

Elfeed offline comes with a tiny Dream based webserver that acts as a proxy in front of the Emacs Elfeed webserver for the API requests. The Dream web server also serves the static assets like the HTML, JavaScript, stylesheets and the Service Worker for the web app. The diagram below shows how data flows between the different components.

+---------------+     +-----------------+
|  Emacs Elfeed |<--->|     Dream       |
|     Server    |     |   Web-server    |
+---------------+     +-----------------+
                               ^
                               |
                               v
 +------------+        +----------------+       +---------+
 |    Web     |        |    Service     |       | Browser |
 |  Client    |<------>|     Worker     |<----->|  Cache  |
 +------------+        +----------------+       +---------+

The Service Worker intercepts all the GET requests from the client and responds using a cache-first-with-cache-refresh strategy. This makes the reading experience feel very responsive. The web client also implements a “good enough” clone of the original search functionality of Elfeed to allow searching and filtering content while offline.

The web app also lets me mark entries as read or star them when I’m offline. The Service Worker caches these operations and updates the server when the server becomes reachable. There’s no smart conflict resolution here, and the API requests just overwrite and update the state on the server.

I have been using this for a couple of weeks now, and am quite happy with how responsive the UI feels and the number of posts I managed to read so far.

But, there are definitely some rough edges to smoothen out.

It’s a pain to make sure that both the servers are running when I’m online and want to sync new updates to my phone. I’m considering adding a tiny Emacs helper to manage this.
I need to remember to open the app on my phone to trigger a cache update before stepping away from my laptop. I want to explore using the Background Sync API (not available on Firefox) to possibly make this easier.
The UI could use some improvements to indicate pending syncs when offline and also indicate the last sync with the server to help ensure everything is synced before stepping away from my laptop.
Service workers need the server to be accessible via https. I have a wrapper script around mkcert to make this easy, but some apps on my phone refuse to work correctly when there are user installed certificates.

Why OCaml?

At work, I write OCaml every day and enjoy it. This project seemed like a good one to try out js_of_ocaml since everything is local and I don’t need to worry about bundle sizes, etc.

js_of_ocaml makes it quite convenient to share code between the server and the client side. And given there’s also a “third” Service Worker component in the mix, I thought it would make life easier. For instance, the message types and the serialization code for the messages between the client and the service worker are shared.

I’m also using this project to experiment with Dune’s “soon to be released”™ package management that our team at Tarides is working on building.

How can you use it?

You can try out Elfeed offline’s UI here as a static site (with some posts from Planet Emacslife and The OCaml Planet). You should be able to read posts, search and filter for posts, mark them as read or star them. Any changes you make should be preserved between reloads.

If you are an Emacs user but don’t use Elfeed, you can checkout the Elfeed’s README for instructions to set it up. It also has links to a bunch of blog posts and videos that show off the features of Elfeed.

If you are already using Elfeed, you should be able to set it up quite easily if you’re happy installing some OCaml tooling. The installations instructions are available in the README.

Outro

I have been enjoying using Elfeed offline for the past couple of weeks. And I’m hoping I’ll end up spending much more time reading in it, than fixing and building it. It currently definitely feels like something I built for myself, but I’d love to hear from others who try it out.

Ocaml on Noetic Nought