Motivation & Complexity
Inline Validation
Validate a field when the user leaves it (on blur), not on every keystroke and not only at submit. Clear that field's error the instant the user edits it again.
Mechanism
Why it happens
A field's value is provisional until the user signals they're done with it by moving on, so validating earlier judges an answer before it's finished. Waiting until submit delays the verdict until every field's errors land on screen at once, forcing the user to hunt across the whole form for what's wrong. Clearing an error as soon as the field goes dirty keeps feedback matched to reality, since a stale error contradicts input the user has already changed.
Impact
Why it matters
- Per-keystroke validation flags an in-progress answer (an email half-typed) as wrong before it's finished
- Submit-only validation surfaces every error across a long form at once, forcing the user to hunt for what to fix
- An error that lingers after the user has already corrected it reads as a bug, not as feedback
Example
Without vs. with
Enter a valid email address.
The email field shows "Enter a valid email address" after the very first keystroke, and the message stays on screen even after the user finishes typing a valid address.
Checked when the field loses focus
The email field stays quiet while the user types, validates once they tab away, and clears the instant they start correcting it.
Checklist
How to apply it
Validate a field on blur, the first time focus leaves it, not on every keystroke
Clear a field's error the moment the user edits it again (marking it dirty), even before the next blur
Don't withhold all validation until submit; blur validation should already have caught most problems by then
Anchor each error to its own field instead of batching every error into one top-of-form summary
Where it shows up