“Designing a data-intensive Web site amounts to specifying its characteristics in terms of various orthogonal abstractions, each captured by a distinct model. The structure, composition, navigation, and presentation models enable the description of read-only web sites. They can be extended to cope with the specification of content management and integration with external services, through the addition of operations, which can be defined and added to the hypertext model. They are invoked as a side effect of navigation and permit one to specify commonly found interaction patterns as data entry, personal data management, and shopping carts.” (About WebML.org)