The anatomy of a credit card form
> “My card number has spaces. Do I enter my card number with spaces, or without?” To solve for this, we limit the input values to numbers only, so 0–9. So if a user types a space, it does not register and it does not affect the number format.They failed, at the most important part of the form. The correct, i.e. the most user friendly solution is, obviously, to allow spaces (and optionally other characters, like "-,._") but ignore them when submitting the form. That way, users can type in the number any way they want. Using spaces to group digits makes it much easier to check the number after you've entered it (not that it's necessary, but I still often do it).Edit: they seem to stick to this failure mode with other inputs as well, e.g. with the postal code input. In almost all circumstances, it's better to allow users to enter anything, and give feedback on whether it's correct or not (either when the focus moves to the next input, or better yet in realtime when the user is entering information, as long as you make sure that partially entered, potentially correct information is not labelled an error). For bonus points, reformat the entered information to some standard format (e.g. grupped dogits for card number) after the focus moves away.