Also known as Robustness Principle, which comes from the creation of the internet, Postel’s Law says “Be liberal in what you accept and conservative in what you send”
In simpler terms we should be forgiving of user input, such as if they make a small mistake, like typos, formatting errors etc, your app should still understand and work for them. On the other hand being strict with output is also improtant. The system should always provide clear, correct and predictable responses.
As designers we should be empathetic to, flexible about, and tolerant of any various actions the user could take or any input they might provide. If we follow Postel’s Law we can reduce user frustration, makes technology feel more human and improves the trust between users and the product.