Firefox is in the process of shipping a new implementation of CSS styling written in Rust as part of Mozilla’s Servo project. Firefox has 20+ million lines of code and hundreds of millions of users, so this is no small undertaking! As a case study for integrating a large, multi-repo Rust project into a larger C++ project, this talk explains how we: - created bi-directional FFI boundaries that maximized each project’s strengths - addressed cross-language mismatches including threadsafety and ownership - integrated two fast-moving codebases while managing regressions We’ll cover specific successes and failures that emerged over the course of the project, and discuss how Rust contributed to those outcomes.