However, in this sort of application, the lines between style and content start to blur a little. They could be unblurred in a concrete application by only allowing reference elements within events. Conversely, you could define the panel content language as a series of event schedules that contain their content directly. But, the details of the presentation of each panel element, such as border size, color, etc., would be purely a matter of style. I was rather surprised at how easy it was to type in the event schedules above.
Regions are explicit coordinate spaces.
Finally, note the indirection in all of this. This indirection is crucial both to the implementation and the flexibility of the system. It is the indirection of pointing from an event schedule to the content that makes the system flexible and maintainable. It only requires the ability to create processors that create from the source a runtime object that expresses the structures in the source. In the future, the binding will be as late as possible using true HyTime engines that interpret the source and create the presentation directly at presentation time.