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Try to validate this url: http://www.provincia.padova.it/agriturismi/portale/ricerca.aspx by "Validate by url" and after try to validate the same page by "Validate by direct input". Validator shows different results.
The difference comes from the fact that your asp.net server sends different content when requested by a browser, or requested by the validator for validation. Use "show source" when you validate by URI and direct input, the differences are flagrant.
*** Bug 4759 has been marked as a duplicate of this bug. ***