Error validating application
If you want to allow any kind of letters (not only A-Z) you're looking for the letternumeric validator.Validate qty of checkboxes in a group (same name) have been checked, using min, max or range.Return false to prevent the default behaviour when adding/removing error messages.
Only the first checkbox element in the group needs to have the validation attributes added.
You can use the setup function if you want to change the border color to something else, set the property to an empty string if you don't want the border color to change when the input has an invalid value.
The If wanting to use a custom method that handles the displaying/removing of error messages you should use the two methods described above.
If you don't want this feature you can read the setup guide on how to disable it.