Use Inline when
One field is wrong, or you can check it as people leave the field. The fix sits right beside the problem.
Signs it fits
- A single field has the problem
- Validation when leaving the field
- Short forms
- Format hints for dates and phone numbers
Use Summary when
Submitting finds several problems at once, often in a long form. A summary at the top counts them and links to each field. Keep the inline messages too.
Signs it fits
- Several errors after submit
- Long or multi-section forms
- Checks that run on the server
- Keyboard and screen reader users need one place to land