Input assistance
Placeholders cannot replace labels. Users must be able to read the field's label even after they start typing.
Do not use placeholders instead of helper text for field restrictions (e.g., character limits, date formats). Users must be able to read these rules even after they start typing.
While often useless, placeholders can serve as examples, e.g. 'Smith' in a 'Surname' field - to help non-native speakers distinguish it from a 'Name' field.