Postel’s Law

Postel’s Law: Designing Robust and Flexible Systems for Better User Experience

Postel’s Law, also known as the Robustness Principle, is a guideline for software design that emphasizes flexibility and robustness in handling input while maintaining strict standards for output. In UX design and development, adhering to Postel’s Law can enhance the usability, compatibility, and overall user experience of digital products.

What is Postel’s Law?

Postel’s Law, formulated by Jon Postel, states: “Be liberal in what you accept, and conservative in what you send.” This means systems should accept a wide range of inputs without error while producing outputs that strictly adhere to defined standards. The principle encourages creating systems that are tolerant of variations and errors in input but produce reliable and predictable outputs.

Importance of Postel’s Law in UX Design

  1. User-Friendly Systems: By accepting a wide range of inputs, systems can accommodate diverse user behaviors and reduce the likelihood of user errors.
  2. Compatibility: Ensuring outputs adhere to strict standards enhances compatibility with other systems, devices, and platforms, providing a seamless user experience.
  3. Error Handling: Robust error handling improves reliability, as systems can gracefully manage unexpected or incorrect inputs without crashing or causing significant issues.
  4. Interoperability: Adhering to standards for outputs ensures that systems can interact effectively with other software and hardware, promoting interoperability.
  5. User Satisfaction: Tolerant and robust systems that minimize disruptions and errors contribute to higher user satisfaction and trust.

Key Principles of Postel’s Law in UX Design

  1. Flexible Input Handling: Design systems to accept a variety of input formats and handle deviations gracefully. For example, form fields should accept different date formats and auto-correct common errors.
  2. Strict Output Standards: Ensure that all outputs, such as data formats and API responses, conform to strict standards to maintain compatibility and reliability.
  3. Comprehensive Error Messages: Provide clear and informative error messages that help users understand what went wrong and how to correct it.
  4. Validation and Sanitization: Implement robust validation and sanitization mechanisms to process inputs securely and effectively, preventing issues like SQL injection or cross-site scripting.
  5. Backward Compatibility: Maintain backward compatibility in updates and new versions to ensure that existing integrations and user workflows are not disrupted.

Best Practices for Implementing Postel’s Law

  1. Input Flexibility: Design input fields to be forgiving of user mistakes. For example, a phone number field should accept various formats and convert them to a standard format internally.
  2. Consistent Output: Ensure that all data outputs adhere to a consistent format and standard, such as using ISO 8601 for dates and UTF-8 for text encoding.
  3. Graceful Degradation: Implement graceful degradation to ensure that if certain features fail or are unsupported in a user’s environment, the system still remains functional.
  4. Extensive Testing: Perform extensive testing with a wide range of inputs to ensure the system can handle unexpected or incorrect data gracefully.
  5. Clear Documentation: Provide clear documentation for both input requirements and output formats to guide developers and users in integrating and using the system effectively.

Tools for Implementing Postel’s Law

  1. Form Validation Libraries: Use libraries like Yup, Joi, or Validator.js to implement robust form validation that can handle a variety of input scenarios.
  2. API Documentation Tools: Tools like Swagger or Postman help ensure that API outputs conform to specified standards and are well-documented for developers.
  3. Automated Testing: Use automated testing tools like Jest, Mocha, or Selenium to test a wide range of inputs and verify consistent outputs.
  4. Error Monitoring: Implement error monitoring tools like Sentry or LogRocket to track and analyze input-related errors, helping to refine and improve input handling.
  5. Data Sanitization Libraries: Use data sanitization libraries to clean and validate inputs, ensuring they are secure and standardized before processing.

Real-World Examples

  1. Web Browsers: Modern web browsers implement Postel’s Law by being liberal in interpreting HTML, CSS, and JavaScript, handling minor errors or deviations gracefully while generating standards-compliant code for output.
  2. Email Clients: Email clients often accept a variety of email formats and handle deviations in MIME types, character encodings, and attachment formats while sending emails in a standardized format.
  3. API Design: RESTful APIs that accept flexible input data (e.g., different date formats) and return standardized JSON responses demonstrate Postel’s Law by ensuring compatibility and usability.
  4. Payment Systems: Payment processing systems that accept various input formats for credit card numbers, addresses, and currencies but output standardized transaction data help ensure smooth transactions.
  5. Content Management Systems (CMS): CMS platforms like WordPress or Drupal that allow diverse input formats (e.g., rich text, markdown, HTML) and produce standardized output (e.g., HTML5) adhere to Postel’s Law.


Postel’s Law is a valuable principle in UX design and development, emphasizing robustness and flexibility in handling inputs while maintaining strict standards for outputs. By adhering to this principle, designers and developers can create systems that are user-friendly, compatible, and reliable. Implementing best practices and leveraging the right tools ensures that digital products can accommodate diverse user behaviors and maintain high standards of performance and interoperability.

Ondrej Zoricak
Ondrej Zoricak