Ben Lynn's page https://crypto.stanford.edu/~blynn/compiler/ compiles (a large subset of) Haskell to web assembly (which you can download; a prime number sieve yielded 40KB of code) and runs it in the browser.

you can just compile c/c++ to wasm in the browser - there are wasi/emscripten builds of clang itself around (yosys, clang-repl, etc).

C based Mandelbrot WASM demos can be ~1 KB total. Assuming you mean a simple scripting language though, Assembly Script does exactly that.

I'm working on a TypeScript/Swift/Dart style language, and currently this hello-world is 1444 bytes:

    export let main = () => {
      console.log("Hello, World!");
    };

I'm trying to make that smaller. The binary includes the Console class, which is needed (I may be able to tree-shake the non log() methods away), but also the Error and IndexOutOfBoundsError classes which aren't needed because there are no catch() expressions.

I think it really helps to have a language designed from the ground-up to obsess over bytes for WASM. Trying to do that with a familiar high-level language with a rich standard library is tricky.