Resilience

Postel's Law

Be liberal in what you accept

Key Insight

Be conservative in what you send, liberal in what you accept—design interfaces that handle variable user input gracefully.

Key Points

  • •Be conservative in what you send, liberal in what you accept—handle variable user input gracefully
  • •Accept diverse input formats (dates, phone numbers, addresses) without requiring exact formatting
  • •Output consistently formatted, well-structured data regardless of input variation
  • •Anticipate typos, extra spaces, mixed cases, and partial entries
  • •Parse user intent rather than demanding syntactic precision
  • •Reduce friction by adapting to how users naturally behave
  • •[FIGURE 5-1 — Face ID flexibility] Shows: iPhone Face ID authentication screen. Teaches: Face ID accepts variations in appearance (glasses, hats, facial hair changes) while maintaining security—liberal in what it accepts. Reproduce as: Illustration of varied face inputs → single authenticated output.
  • •[FIGURE 5-2 — Responsive web design] Shows: Same website displayed across desktop, tablet, and mobile viewports. Teaches: Responsive design embraces the fluid nature of the web—accepting varied viewport sizes and outputting appropriate layouts. Reproduce as: Multi-device preview showing layout adaptation.
  • •[FIGURE 5-3 — Progressive enhancement search] Shows: Search component with basic text box that enhances to include autocomplete on capable browsers. Teaches: Progressive enhancement accepts varying browser capabilities while providing the best possible experience for each. Reproduce as: Side-by-side basic vs enhanced component.
  • •[FIGURE 5-4 — Design systems scaling] Shows: Design system components used consistently across multiple products. Teaches: Design systems enable consistent output across varied inputs (different teams, products, platforms). Reproduce as: Component library → multiple product implementations diagram.
  • •[FIGURE 5-5 — Text expansion i18n] Shows: English text "Search" expanding to Italian "Cerca" with different lengths. Teaches: Interfaces must accept varied text lengths across languages—design for expansion and contraction. Reproduce as: Multi-language string length comparison. Data: English→Italian expansion example.
  • •[FIGURE 5-6 — Amazon font scaling] Shows: Amazon.com adapting to user's minimum font size accessibility setting. Teaches: Accept user accessibility preferences and adapt layout accordingly—don't break when users customize their experience. Reproduce as: Same page at default vs large font size.

Guidelines

  • ✓Accept phone numbers in any format: (555) 123-4567, 555-123-4567, 5551234567
  • ✓Parse dates flexibly: 1/15/24, Jan 15, 2024, 2024-01-15 should all work
  • ✓Trim whitespace, normalize capitalization, and strip special characters automatically
  • ✓Use input masking as a guide, not a gate—don't reject partial input
  • ✓Show formatted output alongside raw input so users learn the expected format
  • ✓Reserve errors for genuinely uninterpretable input, not formatting differences

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