Be liberal in what you accept, and conservative in what you send. Jon Postel articulated this principle in RFC 793 (1981) as a robustness guideline for TCP implementations: systems should handle any reasonable variation in input gracefully while producing precisely correct output. Applied to user interfaces, it translates directly: accept the many ways users naturally express input, normalise it to the system's requirements internally, and return output that is clear, consistent, and correct. Do not punish users for deviating from your expected format.
Postel's Law
Why accepting flexible input and producing clear output builds better forms
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Designers Academy
3 December 2025 · 3 min read
UXPrinciples
In brief
Postel's Law, when applied to user interfaces, advocates for systems that gracefully accept many forms of user input while consistently producing clear, correct, and actionable output.
