Form validation is the check performed on entered data before submission. How it is handled significantly affects how many people complete the form. It is good practice to check fields continuously while they are being filled in, not only after submission, so that the person does not have to search the entire form for the error. The error message should appear next to the specific field, be clearly worded and state what the user should do, not merely that the input is incorrect. Data formats should be accepted in the forms people commonly write them – a phone number with or without spaces – and adjusted automatically rather than rejected. Required fields should be kept to a minimum and clearly marked. A clear confirmation must follow submission. It is worth going through the entire form on mobile every so often, where most problems with filling it in show up most clearly.
See also: Form optimisation, Microcopy, Error states and empty pages.
