1. One or more computer-readable media, comprising: a data structure configured to represent a pair of interfaces for implementing an extended rendering behavior of an external component that participates in rendering with a comparable rendering behavior of a core engine via a mechanism through the pair of interfaces, the pair of interfaces comprising: a first interface returnable for the extended rendering behavior by the external component for calling by the mechanism, the first interface configured to provide at least a drawing method to request that the external component perform drawing rendering capability, and an initial method to request that the external component specify how the extended rendering behavior is to participate in rendering with the comparable rendering behavior of the core engine; and a second interface exposable by the mechanism for calling by the external component during participation in rendering, the second interface configured to provide at least an invalidate rectangle method to specify that a rectangle rendered by the external component is out-of-date, and an invalidate region method to specify that a non-rectangular region rendered by the external component is out-of-date.