Enter a credit card number. It should be spaced out into four groups of four numbers. Pure CSS. No JavaScript here.
Lots of people want this sort of thing, but they end up doing it with JavaScript and rewriting the content of the text field as you type into it. Which is a nightmare and means handling the delete key yourself (see Stripe's jquery.payment for a great implementation, but it'd be nice not to have to, right?)
Only seems to work in Chrome/Opera, possibly because CSS columns aren't really designed to apply to textareas. And it's alarmingly sensitive to the size of the font you're using, hence me using a webfont here. Also, iOS Safari deals really badly with it... — @sil