Web resource load blocking API

In various embodiments described herein, a content extension and programming interface enable third-party content extensions to supply transformation and filtering actions and associated criteria to native web clients on a system. In one embodiment, the native web client loads data for a third-party content filtering, blocking, or transformation extension and conducts extension defined actions without requiring additional intermediation by the third-party content. In one embodiment, a third-party application downloaded to a mobile or handheld device can include an extension data object that provides actions and triggers that are loaded by a native web client of the system. The third-party application is not required to execute continuously and can exit or be terminated once the web client retrieves the extension data object.