Checking input well.
WHEN TO VALIDATE
After a field is completed, not while typing.
WHY
An error shown before someone has finished is hostile.
WHEN TO VALIDATE IMMEDIATELY
Once a field has already shown an error, so correction is confirmed.
WHAT TO VALIDATE ON SUBMISSION
Everything, again.
WHAT TO SHOW
What is wrong, specifically, beside the field.
WHAT NOT TO SHOW
A single generic message at the top An error list detached from the fields
WHAT TO ASSOCIATE
Each message with its field, so assistive technology announces it.
WHAT TO DO ON SUBMISSION FAILURE
Move focus to the first field with an error.
WHY
Otherwise a keyboard user must hunt for it.
WHAT TO PRESERVE
Everything entered.
WHY
Losing it is the most damaging form failure there is.
WHAT TO DEFINE ONCE
The validation rules, shared between client and server where the stack allows.
WHY
Divergent rules produce errors the user cannot resolve.
WHAT TO REMEMBER
Client validation is convenience. The server decides.
