Abstract: Systems, methods, and apparatus, including computer program products, for applying style changes to markup language documents. A replacement style sheet parameter value, which is to replace an existing style sheet parameter value that is associated with a previously rendered markup language document, is received and applied to the rendered markup language document. The replacement style sheet parameter value may be included in a style block and inserted into a Document Object Model data structure that represents the rendered markup language document. Some implementations allow the effect of a design parameter change to be previewed immediately, without re-rendering an entire markup language document, regenerating one or more style sheets, or communicating with a server. Parametrized representations of style sheets may include bindings between parameters, which may result in more compact and efficient style sheet parametrizations in some implementations.