BizTalk™ Unleashed
by Susie Adams, Clifford R. Cannon, Dilip Hardas, Cuneyt Havlioglu, Akhtar Hossein, Charles Kaiman, Tom Lake, Bill Martschenko, Rand Morimoto, Robert Oikawa, Rick Pearson, Kevin Price, Stephen Tranchida, Larrry Wall
History of XML Schemas
In Chapter 2, “XML and Related Technologies,” you learned that XML is a subset of SGML. SGML uses something known as a Document Type Definition (DTD), for defining the structure of an SGML application language.
A DTD defines the semantics of an application language—that is, the names of elements and attributes and the markup's structure—and the hierarchy of those elements within the document. For instance, an HTML DTD dictates that the <HTML> element (or tag) must come first in an HTML document, followed by a <HEAD> tag, and then a <BODY> tag. You can also gather from the DTD that inside the <HEAD> element may be a <META> tag with certain attributes, such as a name attribute for defining certain properties of the document ...
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