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Where is the best place to show validation/error messages in a form?
Why do credit card forms ask for Visa, MasterCard, etc.?
It's quite easy to write code to parse CC numbers, grab the type and verify the length and checksum, and even format it live; I just did it a couple months ago. As for your second and third points, it's a much better experience for the user to get live feedback than waiting until he submits the form. A user enters a number starting with amex and too long? Notify them immediately. A user meant to use a visa but enters a MC? The visual feedback of showing a MC logo should tip him off.