The HTTP IETF RFC (2616) states:
> The first digit of the Status-Code defines the class of response. The last two digits do not have any categorization role. There are 5 values for the first digit:
> - 4xx: Client Error - The request contains bad syntax or cannot be fulfilled
What do you think status codes like 404, 405, 406 are for? You say they shouldn't be for "did the application successfully validate the user's input data" but status code 400 is explicitly for bad requests. In your view should a HTTP server ever return 4xx?