webrtc-unreliable VS draco

The Cargo.toml on GitHub has the optional dependency specified correctly.

I use WebRTC for multiplayer, and Rust has a great library (webrtc-unreliable) To be fair, C++ has libdatachannel which I like using too.

You can, in theory, get UDP-like characteristics out of a different web API - with a WebRTC datachannel configured to be unreliable and unordered. I haven't seen anybody use that API just because it's a pretty big pain to use. The developer of Agar IO called it too hard to use in a HackerNews comment, but nowadays there's simpler libraries than the ones he had in 2016 like webrtc-unreliable and libdatachannel. You still need to run a STUN/TURN server and the integration is easier than it was back then, but still much harder than raw UDP sockets though.

webrtc-unreliable: Provides an async API to a UDP-like transport over WebRTC

You can do this today with https://github.com/kyren/webrtc-unreliable

Take a look at webrtc-unreliable. I haven't tried it, but it claims to implement just a simple subset of webrtc to get udp-like functionality.

File size is important on the web, so I wanted to use Draco, which doesn't have good support for Rust (though I'm sure clean bindings are possible).

No direct DOM access is a bit of a pain, you're definitely not going to write your whole app in WASM anytime soon - it makes for decently good modules though, for example the Draco 3D geometry library provides a nice and clean WASM encoder/decoder module.

This is not a direct solution to your problem. But you could use a compression tool like https://github.com/google/draco to reduce the total filesize.