Simon St.Laurent
Dynamic HTML: A Primer / XML: A Primer / Cookies
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1.0 Introduction
In order for document processing to be reliable, it is necessary to be able to
describe classes of documents and to verify individual documents' membership
in these classes -- in other words, to be able to express constraints on
documents and thus define 'document types'. XML inherits a mechanism for doing
this from SGML: the Document Type Definition. XML DTDs can perform a subset of
the functions of SGML DTDs.
DTDs have many well-documented flaws and it is necessary to experiment with
new ideas in schema design. These ideas include a syntax that is more like
that of XML document content, certain kinds of extensibility and a cleaner
separation between parsing and verifying. XSchema is an experimental schema
language designed to provide a starting point for these experiments. So that
XSchemas will be immediately useful with existing software, the XSchema
specification will describe a conversion from XSchema documents to DTDs.
This initial version of the XSchema specification is deliberately simple,
providing an initial base for implementations while introducing as few
complicating factors as possible. Authors accustomed to DTD creation will find
their toolset constricted; it is hoped that supporting software and tools
available from other standards will make up for this reduced toolset.