1. A method comprising, by one or more computer systems: compiling a plurality of possible instances of web pages for presentation eligibility at a position in a web page flow, each web page having possible feature variations; accessing a current-visitor context of a current visitor to a web page in a current web-browsing session, the current-visitor context comprising data associated with the current visitor including background data about the current visitor, the web page including possible feature variations; predicting, by the one or more computer systems, a plurality of responses of the current visitor to at least one of the possible instances of the web page based on the current visitor context, the possible feature variations of the possible instances of the landing page, one or more previous visitor contexts of one or more previous visitors to whom one or more previous actual instances of the landing page have been presented, each of the previous actual instances of the landing page having been constructed for one of the previous visitors based on one of the possible instances of the landing page selected for the one of the previous visitors, and one or more actual responses of the previous visitors to the previous actual instances of the landing page; selecting based on the current-visitor context a particular one of the 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 including the background data about the current visitor; and removing from presentation eligibility a particular one of the plurality of possible instances of the web page being substantially least likely to generate a highest expected outcome.