When HTTP/2 came out, I was wondering how long it would take for the billions of websites/APIs/webservices/... out there to migrate from HTTP/1.1

HTTP/2 came out in 2014, and since then I still haven't seen it used in production in the jobs I worked at.

Now, I wonder again, will HTTP/4 be released before we migrate to HTTP/3 ?

The real question here is, what are the arguments in favor of a migration? When starting a new web project, should you really consider anything else than HTTP/1.1?

If your website has a lot of small files served, H2 and H3 offer dramatic improvements: browser says "gimme those 100 files" and server sends them all over a single connection in a streaming fashion.

Whereas with H1 browser opens up to 6 connections, downloads 1 file on each, then once the response comes it tells "ok now gimme another file" etc. (Another RTT until it comes back). That's many round-trips saved.

This is probably mostly useful when downloading many small images. It can be also useful for splitting JS into smaller bundles to favor cache hits for returning visitors. (An be also useful to serve ES modules without or with minimal bundling; however in practice bundling is still useful given that modern JS apps have 100s/1000s of files and small files overhead still matters at such numbers).

H2 has built-in support for interesting concepts like prioritization and re-prioritization of requests. Unfortunately, most impls in clients and servers are either not full or not very good:

I recommend googling "http/2 prioritization" and checking blogs and talks from Andy Davies and Pat Meenan. The subject is super interesting and inherently hard; browsers need to make certain assumptions whether scripts, fonts or CSS are more important and should be prioritized and it's non-obvious how to handle it best.

https://github.com/andydavies/http2-prioritization-issues

https://medium.com/dev-channel/javascript-loading-priorities...