Slint does not use a browser. Instead, it has its own runtime written in rust and uses a custom DSL to describe the UI.

It has API for different programming language.

For Javascript, it uses node or deno for the application logic, and then spawn the UI with its own runtime without the browser.

In a way it is the opposite which took the JS runtime out of electron to replace it with a Rust API, while Slint keeps the JS runtime but swaps the browser for its own runtime (for the JS dev point of view)