Postel's Law
Be conservative in what you send, be liberal in what you accept. Also known as the Robustness Principle.
When to Use
Accept varied input formats
Design interfaces that can handle different input formats, variations in user behavior, and edge cases gracefully.
Provide clear output
Be precise and consistent in what your interface outputs or communicates to users.
Handle errors gracefully
When users make mistakes or provide unexpected input, handle it gracefully rather than failing harshly.
When to Avoid
Don't be too permissive
While accepting varied input is good, don't accept input that could be harmful or lead to security issues.
Origin
Postel's Law, also known as the Robustness Principle, was formulated by computer scientist Jon Postel in 1981 as part of the Transmission Control Protocol (TCP) specification. The principle states that implementations should be conservative in what they send but liberal in what they accept. In user experience design, this translates to being flexible and forgiving with user input while being precise and consistent in system output.