All the drawing operations have to be delegated as a wasm module cannot perform any i/o without support from the host system. It's not currently possible to write to a SharedArrayBuffer from wasm (should get resolved some day), but if it was then you could write a purely software renderer in Scheme and just send the final bitmap over to JS to render in a canvas. You could do this without SharedArrayBuffer right now but it requires a byte-by-byte copy from wasm to JS which isn't great. I wouldn't recommend this approach anyway because your game wouldn't be utilizing the GPU at all. HTML5 canvas uses hardware acceleration, which is why I think it is a good option for the time being.
In the future, when we can efficiently share blobs of binary data without copying, WebGL/WebGPU will be the way to go. This will open up the path for existing Guile game libraries like Chickadee to run on Hoot.