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.
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. 8. The computer program product of claim 7 , wherein the determining comprises determining that the change in appearance affects only a portion of the user interface, and the updating comprises updating only primitives that intersect the portion of the user interface affected by the change.