[Term Entry] Add Postel's Law

content/uiux/concepts/ux-psychology/terms/postels-law/postels-law.md

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+---
+Title: 'Postel’s Law'
+Description: 'Postel’s Law, or the Robustness Principle, encourages flexible input handling while ensuring predictable output in UX design.'
+Subjects:
+  - 'Web Design'
+  - 'Web Development'
+Tags:
+  - 'Accessibility'
+  - 'Design'
+  - 'UX'
+CatalogContent:
+  - 'intro-to-ui-ux'
+  - 'paths/front-end-engineer-career-path'
+---
+
+**Postel's Law**, also called the Robustness Principle, originates from internet protocol design and is summarized as:
+
+> "Be liberal in what you accept, and conservative in what you send." — Jon Postel
+
+While originally intended for software protocols, this principle is widely applicable to _UX design_. It encourages flexibility and forgiveness in how systems handle user input, while still delivering predictable, high-quality output. It is especially important in forms, input validation, error handling, and accessibility.
+
+## What It Means in UX
+
+In UX, Postel's Law urges designers to:
+
+- Accommodate varied user input formats and behaviors.
+- Handle edge cases gracefully rather than failing rigidly.
+- Provide consistent, accessible feedback without demanding perfection from the user.
+
+This principle reduces user frustration by allowing users to complete tasks even when their input is not perfect — supporting a more forgiving, human-centered experience.
+
+## Benefits of Postel's Law
+
+- **Increased usability and task success rates:** Flexible input handling prevents user errors from turning into blockers.
+- **Greater inclusivity and accessibility:** Supporting different patterns of interaction helps users with diverse needs and tools.
+- **Stronger resilience in edge cases:** Systems are less likely to crash or behave unexpectedly under unusual input conditions.
+- **Higher trust in the interface:** When users feel understood and supported, they're more likely to engage fully with a product.
+
+## When and Where to Apply Postel's Law
+
+Postel's Law is especially useful when:
+
+- **Designing forms or search bars:** Let users enter phone numbers with or without dashes, spaces, or country codes and still validate successfully.
+- **Handling user errors or mistyped data:** Allow minor spelling errors in search fields and suggest corrections instead of showing zero results.
+- **Designing multi-platform interactions:** Recognize inputs from different devices (e.g. touch, keyboard, voice) without rigid expectations.
+- **Building accessible interactions:** Ensure that screen readers, keyboard navigation, and alternate input methods receive appropriate, predictable responses.
+
+## How to Apply It in UX Design
+
+- **Support multiple input formats:** Whether it is date fields, email formats, or file names, accept a range of input styles and normalize them behind the scenes.
+- **Add real-time validation and correction:** Guide users gently with hints or auto-formatting instead of stopping them with hard errors.
+- **Make defaults and fallbacks work intelligently:** If input is missing or incorrect, offer smart defaults or safe assumptions to keep users moving forward.
+- **Use clear, non-blaming error messages:** Errors should inform, not shame. Make recovery easy and empathetic.
+- **Avoid punishing unexpected behavior:** Always consider how your system should respond to unusual but still valid input, and aim for graceful degradation.
+
+![Diagram showing search engine input into multiple suggestive outputs](https://raw.githubusercontent.com/Codecademy/docs/main/media/postels-law-input-flexibility.png)
+
+## Limitations of the Principle
+
+- **Overly liberal input handling can hide issues or create security concerns:** For example, accepting any format without validation can increase the risk of incorrect or malicious input.
+- **Too much flexibility can reduce learnability:** Users may not know what to expect or how to interact consistently if there are no boundaries.
+- **It should never compromise clarity or control:** Flexibility should support users — not confuse them.
+
+## Examples of Postel’s Law in Practice
+
+- **Search bars that autocorrect typos or suggest queries:** Instead of showing “no results,” the system interprets the user’s intent and offers relevant alternatives.
+- **Date inputs that accept multiple formats:** Fields that let users type “August 27,” “27/08/2025,” or “08-27-25” without error.
+- **Phone number fields that strip spaces or symbols:** Systems that normalize input like “(123) 456-7890” into a clean format instead of blocking it.
+- **Form validation that corrects capitalization or spacing errors:** Automatically formatting names or addresses while still accepting varied input.
+- **E-commerce checkouts that accept minor address variations:** Matching “Street” with “St.” or recognizing mistyped zip codes to assist in delivery.
+- **Search filters that don’t break with wrong case or punctuation:** Recognizing that “UX Design” and “ux design” are the same for query purposes.