8. A computing system comprising: one or more computing devices configured to: cause a change to data accessible via a data model, the data model being usable to create an electronic document page, the page being defined by a page description implemented in a markup language, the page description referring to the data accessible via the data model; create an intermediate representation of the page based on the page description and the data model, the intermediate representation including at least a portion of the data accessible via the data model that includes the changed data, the intermediate representation being renderable to create a rendered page, the rendered page being displayable on a display device; determine an expected value for a portion of the intermediate representation based on the change to the data accessible via the data model; identify an actual value for the portion of the intermediate representation; determine whether the actual value matches the expected value; and when the actual value does not match the expected value, store an indication of an error.
8. A computing system comprising: one or more computing devices configured to: cause a change to data accessible via a data model, the data model being usable to create an electronic document page, the page being defined by a page description implemented in a markup language, the page description referring to the data accessible via the data model; create an intermediate representation of the page based on the page description and the data model, the intermediate representation including at least a portion of the data accessible via the data model that includes the changed data, the intermediate representation being renderable to create a rendered page, the rendered page being displayable on a display device; determine an expected value for a portion of the intermediate representation based on the change to the data accessible via the data model; identify an actual value for the portion of the intermediate representation; determine whether the actual value matches the expected value; and when the actual value does not match the expected value, store an indication of an error. 11. The computing system recited in claim 8 , wherein to create the intermediate representation the one or more computing devices are configured to: execute computer programming instructions included in a page controller associated with the page, the page controller defining one or more operations for creating the page.