On my android with firefox it very was easy to see that the XXXX was an input-box that had to be filled in because each field had a small frame around it, i was surprised to see so many having trouble understanding it. Then i tried in my desktop browser (firefox, chrome and ie) and there there is no frame around the unfocused input fields, the numbers stand alone making it look like a static image of a credit card, except when the fields are focused. This little change made the experience much worse, i suggest always showing the frames around the fields.