Methods, systems, and apparatus, including computer program products for defining landing pages to test different landing page designs. A combination is determined of one alternative version for each of one or more sections in an instrumented landing page. In a client, a landing page variant is generated. Each section of the landing page variant is the corresponding alternative version of the section defined by the combination. The landing page variant is generated in the client by determining for each section whether to display an additional version of the section in lieu of the original version of the section. If it is so determined, the additional version of the section is retrieved and inserted into a new location of the landing page variant, and at least one tag is inserted to cause the original version of the section to be disregarded.