📄 The Robustness Principle

The robustness principle, also known as Postel's Law, states: "Be conservative in what you do, be liberal in what you accept from others."

It's a foundational design principle for protocols and APIs. The idea is that if your software is strict about its outputs but lenient about its inputs, the overall system is more interoperable.

However, the principle has come under criticism in security contexts. Lenient input parsing creates ambiguity, and ambiguity is where vulnerabilities live. The HTTP/3 draft authors explicitly rejected Postel's Law in favour of strict parsing for exactly this reason.