Distinguishing between functional tracking domains and nonfunctional tracking domains on a host web page. In particular, a list of known tracking domains that load content into host web pages may be received. This list of tracking domains may include tracking domains that are functional and tracking domains that are nonfunctional. The tracking domains that are functional may be determined by evaluating various behaviors and characteristics of the tracking domains. Once functional tracking domains have been determined, these functional tracking domains may be allowed, and other tracking domains may be blocked from loading content onto host web pages thereby preserving the functionality of the web pages.