Servo is a complete (although WIP) browser engine in Rust. Stylo is a project to plug the style parts of Servo into Firefox, which currently uses the Gecko browser engine. This is part of Mozilla's ongoing efforts to modernize Firefox by gradually rewriting it in Rust.

WebKit is technically only used by Safari. Chrome uses a WebKit fork called Blink, and MS Edge uses its own engine called Chakra.

I assume the above commenter was expressing relief that someone besides Google is making a fast, modern web browser these days.