In case any Stripe folks read this, I tested it with an invalid month (13) and valid year (16,15,13,12 in that order) and valid everything else, and every time it put the red error highlight around the year instead of the month.
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The Year field is specified as YY but it allows you to write up to 4 digits, even though the characters shown are only 3 (the field is not large enough). Tested on Chrome and Firefox on Windows 7.
nb I tried 2015 as year and it was accepted, so I suppose it's ok to specify years in 4digit format
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