Well, it is for now. This isn't yet a concrete plan that Mozilla has, but from what I've read, the Servo developers would definitely also like to write a JavaScript-engine in Rust, simply because of the gain in security.
Also, browser.html already exists in an early form (and is bundled with Servo). It provides a new UI written in HTML, CSS and JavaScript, so it can be rendered by Servo as well.
> Servo developers would definitely also like to write a JavaScript-engine in Rust, simply because of the gain in security.
(Servo developer here)
This isn't really the case. There is interest in doing this, but it's not something we definitely want to do. The problem with writing a new JS engine is that you need to duplicate years of performance tuning so while it might be safe, it would take immense amounts of work to make it efficient. Rust's safety benefits get reduced when you have JITs and all involved, too.
It would be nice to have, sure, but the amount of work in making a usable one is huge.
Also, browser.html already exists in an early form (and is bundled with Servo). It provides a new UI written in HTML, CSS and JavaScript, so it can be rendered by Servo as well.
https://github.com/browserhtml/browserhtml