The project started as SerenityOS, an OS with a browser built in and it was written in modern C++ with safeguards.

Now, the team is converting parts of the browser into Rust with AI. For example they converted their javascript interpreter into Rust. They are doing bit-perfect matching of input and output to achieve feature parity. See https://ladybird.org/posts/adopting-rust/

wait till you see what operating systems are written in!