1. A content modification system comprising: a processor; and a storage device, the storage device including machine-readable instructions imbedded therein to cause the processor to: receive, from a web server, an HTTP request from a user for content; request data capture instructions identifying data to capture based upon the HTTP request; receive the data capture instructions in response to the request for the data capture instructions, wherein the data to capture based on the data capture instructions includes a visitor ID, a query string in the HTTP request, user profile data, and on-line user behavior including data related to online purchases associated with the user; capture the data in accordance with the received data capture instructions; transmit a content modification request message to an optimization service, the content modification request message including the request for content from the user and the captured on-line user behavior; in response to the transmitted content modification request message, receive from the optimization service: a control tree representation of the requested content, wherein the control tree representation includes a plurality of controls representing elements of the requested content; and a modification instruction comprising an indication to modify an attribute of an existing control of the plurality of controls in the control tree representation, wherein the optimization service determines the attribute of the existing control to modify according to the captured on-line user behavior; identify, within the control tree representation, the existing control to be modified; modify the attribute of the identified existing control in the control tree representation to generate a modified control tree representation based upon the modification instruction; convert the modified control tree representation to modified content in a user viewable format; send the modified content in the user viewable format to the user via the web server; determine whether the data capture instructions include instructions to capture subsequent online user behavior after the modified content is delivered to the user; and in response to a determination that the data capture instructions include instructions to capture subsequent online user behavior after the modified content is delivered to the user, continue to capture and store the on-line user behavior.