A proxy server receives from a client device a request for a network resource that is hosted at an origin server for a domain. The request is received at the proxy server as a result of a DNS request for the domain resolving to the proxy server. The origin server is one of multiple origin servers that belong to different domains that resolve to the proxy server and are owned by different entities. The proxy server retrieves the requested network resource. The proxy server determines that the requested resource is an HTML page, automatically modifies the HTML page, and transmits the modified HTML page to the client device.