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Postel's law 0:36
A 36-second explainer of Postel's law, narrated over the same demo you can try yourself.
Try the interactive Postel's law demo
What this video covers
Postel's law, also known as the robustness principle, suggests being conservative in what you send and liberal in what you accept. This means accepting harmless variations - such as spacing, brackets, or case - while validating, normalising, and storing a canonical version. It does not mean accepting ambiguous, unsafe, or semantically invalid input just for tolerance.
In strict mode the field threw your perfectly clear number back for a stray bracket; in forgiving mode it simply took it and tidied it up. Same number, same intent - the only difference was whether the form did the fiddly bit or made you do it. That's Postel's law: strict in what you produce, generous in what you accept.
