In general, one aspect of the subject matter described in this specification can be embodied in a method that includes rendering Hyper Text Markup Language (HTML) content, in an HTML rendering engine, to primitives of a 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.