That 5% of js glue code necessary right now is just monumentally difficult to get rid of, it's like a binary serialization / interface (ABI) of all DOM/BOM APIs and these APIs are huge, dynamic, callback-heavy and object-oriented. It's much easier to have that glue compiler generated, which you can already do right now (you can write your entire web app in rust if you want):

https://github.com/wasm-bindgen/wasm-bindgen https://docs.rs/web-sys/latest/web_sys/

This is also being worked on, in the future this 5% glue might eventually entirely disappear:

> Designed with the "Web IDL bindings" proposal in mind. Eventually, there won't be any JavaScript shims between Rust-generated wasm functions and native DOM methods