XML Companion, The, Third Edition

Introduction

Beyond the basic concepts that replicate the well-known modelling features inherited from XML DTDs, the XML Schema standard includes several entirely new features, and resurrects some of the advanced features found in SGML DTDs. These features give the schema author the ability to:

• define default and fixed element content values

• define a content model that requires all of the referenced elements to be present, but does not dictate the order in which they can occur

• add attributes to simple element types

• explicitly identify element instances that have no value

• define context-specific elements within other elements, possibly with non-unique names, but with different content models and attributes

• specify that elements must have unique identifier ...