Clearly Wasmer wants to do WebAssembly. But for a whole new, general-purpose programming language, what's the case for that target, versus, say, LLVM intermediate representation?
I started making Onyx because I simply wanted to create my own programming language for the fun of it. I choose WebAssembly as the target for my compiler because it was relatively easy to write my own full-stack solution, instead of relying on something like LLVM. That language evolved over the years until I had something I felt was worth sharing. When the Wasmer team reached out to see if I wanted to do a collaboration blog post, I said yes.
The main reason I didn't go with LLVM was simply because I wanted to write everything myself (for fun) and to have a super-fast compiler. LLVM is not the fastest backend, but it is understandable for all the optimizations that it does.
Even pointing to somewhere in the code that I could start poking around would be great.
You could definitely use Onyx to create a modern lisp interpreter that can run in the browser without any dependencies, but it might be worth porting an existing C project if that is your goal. I'm not gonna stop you from making something like that in Onyx tho; it would be very cool!
There is a however possibility for a REPL for Onyx to exist outside of the browser. This is because Onyx has another runtime it can use alongside WASM called OVMwasm. This runtime is entirely custom (you can see the source code in the 'interpreter' folder on the Onyx GitHub), so it could be modified to do non-standard things with WASM, like add new functions dynamically. That would take some work to achieve, but in theory should be possible.