Specializing low-level systems to specifics of the workload they serve and platform they are running on often significantly improves performance. However, specializing systems is difficult because of three compounding challenges: i) specialization for optimal performance requires in-depth compile-time changes; ii) the right combination of specialization choices for optimal performance is hard to predict a priori; and iii) workloads and platform details often change online. In practice, benefits of specialization are thus not attainable for many low-level systems. To address this, we advocate for a radically different approach for performance-critical low-level systems: designing and implementing systems with and for runtime code specialization. We leverage just-in-time compilation to change systems code based on developer-specified specialization points as the system runs. The JIT runtime automatically tries out specialization choices and measures their impact on system performance, e.g. request latency or throughput, to guide the search. With Iridescent, our early prototype, we demonstrate that online specialization (i) is feasible even for low-level systems code, such as network stacks, (ii) improves system performance without the need for complex cost models, (iii) incurs low developer effort, especially compared to manual exploration. We conclude with future opportunities online system code specialization enables.