I'll note that while the code quality in Rust is pretty good (clean/usually commented/well formatted/tested), it's not exactly idiomatic Rust.
This is because Rust as a language has changed a lot, and the compiler still has old code that was written the "old way" and not updated to use better or more idiomatic alternatives (e.g. elision, if let, etc). Servo is in a similar situation. This is slowly improving as we run clippy on Rust and in occasional manual refactorings (for example when sty was renamed to TypeVariants and ty_foo was renamed to TyFoo to be have the correct capitalization -- the old capitalization was years old), but there still is work to do to make it completely idiomatic.
So if you want to learn how to write idiomatic Rust, I would avoid using the Rust repo as a source. Newer repos and new code in the Rust repo is pretty okay, though.
https://github.com/rust-lang/rust