Methods, systems, and apparatus, including computer program products for showing different landing page designs to a user. One method includes displaying an advertisement on a starting page to a user interacting with a client, the advertisement having a hyperlink to an instrumented landing page; in response to the user selecting the advertisement, downloading the instrumented landing page to the client, the instrumented landing page having one or more sections, each section having a corresponding plurality of alternative versions; choosing in the client a combination of one alternative version for each of the one or more sections; generating in the client a landing page variant, each section of the landing page variant being the corresponding alternative version of the section defined by the combination; notifying a server that the instrumented landing page was reached; notifying the server which combination was chosen; and displaying the landing page variant to the user.