Client-side vs server-side AB testing

Traditional AB testing for comparing two (or more) different alternative pages uses client-side testing.

The standard Google Optimize tool uses this technique. The browser executes JavaScript in the web page to determine which variant to serve which may include different headers or imagery.

VWO has this detailed guide to the benefits of server-side testing. They give the example of comparing two different check out flows with server-side testing. That isn’t possible with client-side testing which is limited to cosmetic changes.

Google Optimize 360 (the paid service) offers server-side testing – see Google Dev guide.