I can't find any sections in the book about features that mainly use external (non-Servo-org) crates for the bulk of implementation; I know about taffy, vello and resvg (and gstreamer) but not sure if there are any others that are relevant. I think this is important because it helps to understand, for instance, when the crate's inherent scope limitations or implementation details limit Servo's functionality (e.g. resvg not supporting animation).