Format inputs for them, not after
Phone number fields that don't tell you the expected format produce a mix of (555) 123-4567, 555-123-4567, 5551234567, and +1 555 123 4567. Then validation rejects three of those four formats and the user has to guess which one you want.
Input masks solve this by formatting as the user types. They press 5-5-5-1-2-3-4-5-6-7 and the field automatically displays (555) 123-4567. No guessing. No rejection. No frustration.
Credit card fields benefit even more. A masked field that groups digits into fours and auto-detects the card type (Visa, Mastercard) reduces payment errors significantly. The user can visually verify their number matches the card because the grouping is the same.
Date fields are the third big win. Is it MM/DD or DD/MM? An input mask that shows the format as placeholder text and auto-inserts the slashes removes the ambiguity entirely. The user just types numbers.
Add input masks to phone number, credit card, and date fields. Use a library like Cleave.js or IMask. Measure form error rates before and after.
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