Postel's Law¶

Status: emerging
Last updated: 2026-06-02
Sources: Lawsofux.Pdf
Tags: [ux-design, design-principles, heuristics, postels-law, robustness-principle, input-handling, error-tolerance]

Summary¶

Postel's law, also called the robustness principle, holds that a system should "be conservative in what you do, be liberal in what you accept from others" (Yablonski, 2024). Applied to UX, it means accepting user input in whatever varied, imperfect form it arrives while producing consistent, reliable output — designing interfaces that tolerate human variability rather than demanding precision. Yablonski presents it as a guiding principle for human-centred experiences that must cope with scale and complexity.

Body¶

Context¶

Yablonski (2024), in the Postel's Law chapter of Laws of UX, adapts a principle originating in internet protocol design to the design of human–computer interaction. The chapter states the principle and reframes it as a stance toward accommodating human input. Within this knowledge base the article connects to Usability And User Experience (error tolerance and forgiving design), to User Requirements Methods (understanding the range of real user input), and to the input/error concerns in Human Error And Reliability; it is one of the laws in Laws Of Ux.

Key Points¶

Yablonski states the principle as: be conservative in what you do, be liberal in what you accept from others, describing it as the robustness principle and a guiding principle for designing human-centric experiences that account for both scale and complexity (PDF pp. 73–84, orig. pp. 53–64). The first half — conservative output — calls for predictable, consistent system behaviour; the second — liberal acceptance — calls for tolerating the varied and imperfect forms in which people supply information.

The design implications concern input handling and forgiveness. Interfaces should accept input flexibly (different formats for dates, phone numbers, names) and interpret user intent generously rather than rejecting anything that deviates from a rigid expected form, while still returning clear and consistent responses (PDF pp. 73–84, orig. pp. 53–64). The chapter frames the law as bridging the communication gap between humans and computers by making technology adapt to human behaviour rather than the reverse.

Conclusion¶

Yablonski (2024) concludes that Postel's law asks designers to absorb human variability at the system boundary: be strict and predictable in what the system produces, generous in what it accepts. The result is interfaces that feel forgiving and reduce avoidable user error.

References¶

Yablonski, J. (2024) Laws of UX: Using Psychology to Design Better Products & Services. 2nd edn. Sebastopol, CA: O'Reilly Media. yablonski2024lawsux

Open Questions¶

How far should "liberal acceptance" extend before it introduces ambiguity or security risk in input handling?
How does forgiving input design interact with error-prevention strategies in Human Error And Reliability?