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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 - the robustness principle - says be conservative in what you send and liberal in what you accept. Take input in whatever shape people naturally give it, then tidy it up yourself, instead of rejecting anything that isn't already perfectly formatted.
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.
