While the definition is ever expanding, cross script scripting (XSS) attacks are generally considered a type of injection problem where malicious input is injected into an otherwise trusted web page causing an unexpected behavior such as sending data to or from an unknown third party web site (cross site).
Most reputation services for identifying malicious sites relies on a large install base of deployed appliances that report new URLs and their associated behavior back to a central database for automated inspection backed up by teams of researchers for those sites that defy automated analysis.