In recent years CSS has become closer to a full programming language through experimental features, for example in 2025 they added if statements and some math functions like modulo

https://www.simplethread.com/new-and-upcoming-css-features-i...

The only thing missing is the ergonomics of a real programming language.

I like the declarative nature of it. It makes it so easy to debug anything, with "simple" introspection tools. I feel that many horrors will be created when we introduce control flow to CSS.

And this project kinda show how far you can go, still, if you really want it :D

A functional language should be able to do both.

It's not just closer. Someone wrote an x86 emulator with CSS (it uses JS only for clock to make it more reliable). https://lyra.horse/x86css/ . So, CSS is officially Turing complete (which is a bit scary IMHO).

BB(5) was discovered last year, so why is this paper coming out now? Is there a new development?

This is the (pre-print) paper produced from that project - i.e. it is a self-contained, complete description of the work completed so that it can be reviewed, and then cited.

The underlying result has not changed, but the presentation of the details is now complete.