What you need to know
A message like invalid input does not help someone recover. Clear field-level feedback reduces repeated guesswork and makes it easier for assistive technology users to locate the problem.
How to apply it
Use visible text near the field, connect it with aria-describedby, mark invalid controls, and summarize errors at the top for long forms. Preserve valid values after submission.
Common mistake to avoid
Do not rely on red borders alone. Include words that explain the problem and a correction when the rule is not obvious.