1. A computer, comprising: an I/O interface; and a processing element connected to the I/O interface and implementing a runtime environment, the processing element also implementing a rendering application that causes the processing element to provide a proxy object used to expose a rendering API not natively supported by the runtime environment, the rendering API invocable by code comprised in a markup document accessed by the processor over the I/O interface, the rendering API exposed by causing the runtime environment to respond to properties, methods, and behaviors defined by the rendering API and mirrored by the proxy object.