1. A proxy server apparatus comprising: a proxy server module adapted to, in a case where a client device has generated a first HTTP GET request for a first URL, receive, from a web server associated with the first URL, a HTTP response corresponding to the first URL; and a Javascript obtaining module adapted to obtain, from Javascript in the HTTP response, modified Javascript, wherein at least one of the following conditions is satisfied: (a) said Javascript obtaining module is adapted to obtain as the modified Javascript, when the Javascript in the HTTP response, upon execution, uses a first random number algorithm in an instance, Javascript that when executed instead uses in that instance an algorithm different from the first random number algorithm; (b) said Javascript obtaining module is adapted to obtain as the modified Javascript, when the Javascript in the HTTP response, upon execution, uses a first chronological algorithm in an instance, Javascript that when executed instead uses in that instance an algorithm different from the first chronological algorithm; or (c) said Javascript obtaining module is adapted to obtain as the modified Javascript, when the Javascript in the HTTP response, upon execution, uses an algorithm to generate a URL in an instance, Javascript that when executed instead uses in that instance a different algorithm to generate a URL, wherein a memory and a processor are included in the proxy server apparatus.