Postel's Law advises, "Be strict in what you produce, and liberal in what you accept." In design, this principle means being rigorous with the consistency and clarity of output while remaining flexible with user input. Essentially, the aim is to handle user errors gracefully while providing intuitive feedback. If users make common mistakes — like missing "www" in a URL or using a non-standard date format — the system should accommodate these errors and still deliver the desired experience.

Here are some ways to apply Postel's Law to design:

• Form validation: Accept diverse data formats (like dates and phone numbers) and guide users through corrections with helpful error messages.
• Password security: Offer immediate feedback on password complexity, encouraging users to create secure yet memorable passwords.