1. A method comprising: receiving, from a first client computer, a first request for a first set of instructions, wherein the first set of instructions define one or more objects, and the one or more objects define a first object with a first identifier; in response to the first request; rendering a second set of instructions by assigning a second identifier to the first object, wherein the second set of instructions is different than the first set of instructions, and wherein the first identifier is different than the second identifier; sending the second set of instructions to the first client computer; receiving, from a second client computer, a second request for the first set of instructions; in response to the second request: rendering a third set of instructions by assigning a third identifier to the first object, wherein the third set of instructions is different than the first set of instructions and the second set of instructions, and wherein the third identifier is different than the first identifier and the second identifier; sending the third set of instructions to the second client computer; wherein the method is performed by one or more computing devices.