The main idea compared to stdweb is to have the ability to call into javascript dynamically but without having to write actual embedded javascript.

From what I've seed stdweb exposes a js! macro with some features to conveniently pass around rust values back and forth.

Also from my understanding of it stdweb also features some abstraction layers of common js APIS based internally on that js! macro.

Ultimately I'd say compared to stdweb, wasm_val aims to provide access to javascript via an API as compared to writing actual embeded js like stdweb does or by writing or generating bindings as wasm-bindgen does.

Edit: Also since wasm_val encapsulates js access in an API it provides additional type-safe guarantees (such as when coercing a JsValue into a rust type)

Also wasm_val does this dynamically, so you could potentially do some stuff during runtime that you couldn't in stdweb due to it using code generation as well.

Hope this answers your question.