Laws of UX
Postel's Law
Be liberal in what you accept, conservative in what you send. Accept varied input formats (dates, phone numbers, spaces in card numbers). Output clean, consistent, predictable interfaces.
In plain terms
Be forgiving about how people enter information — spaces in card numbers, different date formats — and keep your own output clean and clear.
Why this matters
Being strict about input format punishes people who enter data differently — with a tremor, a screen reader, or a password manager. Accepting varied input removes a needless barrier.