XML is a markup language that allows you to create other markup languages. The process of establishing the structure of XML-based markup languages is known as data modeling. The resulting data model for a custom markup language is defined in a construct known as a schema. Two primary approaches exist for creating schemas for XML languages: DTDs and XSDs. There are advantages and disadvantages to each approach, but they are both suitable for most XML applications.
In this hour you gained some important insight into document validation and found out what it means for a document to be well formed. You also explored the details of the DTD approach to defining XML schemas, and found out how DTDs are responsible for describing the structure and format of a class of XML documents. You saw that a DTD consists of markup declarations that determine the rules for a custom markup language. In addition to providing a formal set of rules for a markup language, DTDs form a critical part of XML in that they provide a means of validating documents for accuracy. The hour culminated with the creation of a complete DTD for the Endurance Training Markup Language (ETML). You also saw a sample XML document created in ETML.