Craft Fundamentals · heuristic
Postel's Law
Playable Play this law (~3 min)or keep reading below ↓
Be liberal in what you accept, conservative in what you send. Accept every reasonable way a human might type a date, phone or name — then store and display one clean canonical form. Rejecting valid effort on a technicality is a design bug.
Where it comes from
Jon Postel wrote the robustness principle into the 1980 TCP specification (RFC 761) as an engineering rule for the young internet: implementations should tolerate imperfect input from others while emitting strictly correct output themselves. The internet's survival through decades of half-broken software is the proof. Forms inherited the lesson: the machine should absorb messiness, not the human.
Origin: Postel's robustness principle, TCP spec RFC 761 (1980)
In the wild
- Phone inputs
Great forms accept +91 98765 43210, 9876543210 and (987) 654-3210 — and never show a red error for a space.
- Search boxes
Tolerating typos, plurals and word order ("shoes red" = "red shoe") is Postel applied to language.
- Payment forms
Card fields that auto-strip spaces beat ones that scold users for pasting their own number.
Further reading
Don't just read it — feel it.
Play the 3-minute game and let your own reactions prove the law.
Play the lesson