Server side web based ad stitching and user specific optimization are disclosed. In various embodiments, a request for a page is received via a communication interface. A processor is used to provide an optimized version of the page at least in part by determining dynamically an optimized content element for the page based at least in part on a user attribute associated with the request and content performance data generated based at least in part on testing of alternative versions of the page with respect to other users having the user attribute.