1. A computer-implemented method, comprising: receiving at a proxy server, from a computing device, a first request for first content located at a content provider, the first request including a test identifier; accessing a testing library for a test script associated with the test identifier; transmitting the first request from the proxy server to the content provider, the first request excluding the test identifier; receiving, from the content provider in response to the first request, the first content including a reference to the content provider; modifying, at the proxy server, the first content to generate second content, wherein the modifying comprises an insertion of the test script and a replacement of the reference to the content provider in the first content with a reference to the proxy server; and transmitting the second content from the proxy server to the computing device.