1. A method comprising: intercepting, from a server computer, a first set of instructions that define a user interface; executing, using a headless browser, the first set of instructions without presenting the user interface; rendering a second set of instructions, which when executed by a client application on a client computer, cause the client computer to present the user interface, wherein the second set of instructions are different than the first set of instructions; sending the second set of instructions to the client computer; wherein the method is performed by one or more computing devices.