JS has little to do with accessibility. Most web accessibility comes from the Dom and aria attributes as well as semantic tags. You can do all of that with wasm too.

Are you asking about how it will work with wgpu based applications? This will work the same as it does on desktop applications. The program calls out to libraries that support talking to screen readers. I know rust the language with the best support for and ecosystem around wasm libraries like this already exist and ui frameworks like egui already have some support built in.