Something that addresses your concern is coming post 1.0. Servo needs something like inheritance to model the DOM efficiently. It may or may not end up looking like inheritance, though. We determined that our possible solutions are backwards compatible, so we are doing it post 1.0, though.

(Oh, and Rust doesn't get everything Servo needs, what I mean to say is "Servo has demonstrated that there is real-world need for something like inheritance, and so we will add it.")