[Term Entry] Add Postel's Law

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

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+---
+Title: 'Postels Law'
+Description: 'Postels Law, also known as the Robustness Principle, encourages systems to accept input flexibly while remaining strict in their output — a principle that applies to UX and interface design for resilience and accessibility.'
+Subjects:
+  - 'Web Design'
+Tags:
+  - 'UX'
+  - 'Design Principles'
+  - 'Accessibility'
+CatalogContent:
+  - 'intro-to-ui-ux'
+  - 'paths/front-end-engineer-career-path'
+---
+
+**Postels Law**, or the _Robustness Principle_, originates from early internet protocol design and states:
+
+> 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, Postels 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
+
+The principle helps reduce user frustration by allowing users to complete tasks even when their input is not perfect, supporting a more forgiving, human-centered experience.
+
+## Why It Matters
+
+People do not always interact with interfaces in the exact way designers expect. If a system requires perfection, users may hit frustrating dead ends. Thus, designing with Postels Law in mind means anticipating human variability and embracing it.
+
+### Benefits include
+
+- _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 are more likely to engage fully with a product.
+
+## When and Where to Apply Postels Law
+
+Postels 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 flexible input formats converging into a standardized system output](https://github.com/raw/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.
+
+Use Postels Law to build more resilient, forgiving, and user-friendly experiences — while still being mindful of context, constraints, and risk.