Flexible information appearance and behavior can be achieved with an extensible multi-phase transform engine. The transform engine can apply a series of transforms to markup language content. In a help service embodiment, application creators can specify a Uniform Resource Indicator (URI) for help content, and a collection of transforms and/or executable code for formatting and presenting the help content. The transforms may be custom transforms registered with the help service, or may be selected from a set of stock pre-registered transforms. The transforms can be applied by the engine to the help content. Custom transforms and transform selection may be used to control content appearance and engine output format. Transform operation may also trigger custom executable code to accomplish desired help behaviors.