9. The computer program product of claim 7 , further comprising bridging between a first script engine used by the HTML rendering engine and a second script engine used by the vector graphics rendering engine, the bridging comprising generating objects for the first script engine that represent objects in the second script engine and generating objects for the second script engine that represent objects in the first script engine.
7. A computer program product, encoded on a computer-readable medium, operable to cause data processing apparatus to perform operations comprising: rendering Hyper Text Markup Language (HTML) content, in an HTML rendering engine, to primitives of a separate vector graphics rendering engine; rendering the primitives, in the vector graphics rendering engine, to provide a user interface; receiving an input event via the user interface; determining, in the HTML rendering engine, a change in appearance for the user interface based on the input event; and updating at least one of the primitives for rendering by the vector graphics rendering engine in accordance with the change in appearance to update the user interface.