1. A method comprising, by one or more computer systems: accessing a current-visitor context of a current visitor to a web page in a current web-browsing session, the current-visitor context comprising one or more data associated with or concerning the current visitor; and selecting based on the current-visitor context a particular one of a plurality of possible instances of the web page for presentation to the current visitor, the particular one of the plurality of possible instances of the web page being substantially most likely to generate a highest expected outcome from interaction with the web page by the current visitor as indicated by the current-visitor context.