WebRender is described as 'is Mozilla's GPU-based renderer for web content used in Servo and built with Rust' and is an app in the Development category. There are four alternatives to WebRender for Linux, Mac and Windows. The best alternative is Sauron native, which is both free and Open Source. Other great apps like WebRender are OrbTk (Free, Open Source), Azul.rs (Free, Open Source) and Conrod (Free, Open Source).
The Orbital Widget Toolkit is a multi platform (G)UI toolkit for building scalable user interfaces with the programming language Rust. It's based on the Entity Component System Pattern and provides a functional-reactive like API.