XML (Extensible Markup Language) is a markup language that defines a standard way to add metadata to documents. It is designed to be simple, open, and extensible. XML separates document content from presentation, allowing the same content to be presented in different ways. XML uses tags to delimit elements in a document and organize them in a hierarchical tree structure. The XML specification defines rules for creating valid XML documents that can be processed and shared.

1. Introducing XML
PART 1

2. "XML", as we all know, is an acronym for Extensible Markup Language.
Markup is a method of conveying metadata (information about another dataset).
SGML-based markup languages all use literal strings of characters, called tags to delimit the
major components of the metadata, called elements.
SGML is an acronym for Standard Generalized Markup Language, an older and more much
complex markup language than XML. It has been codified as an international standard by the
ISO (International Organization for Standardization) as ISO 8879 and WebSGML.

3. Separation of Semantics and Presentation
XML is all about the description of data, with nothing said about its presentation.
HTML combines some rudimentary descriptive markup, plus a great deal of markup
that describes the presentation of the data.
The initial layer of XML processing is the XML parser, which can optionally use a DTD or schema
to describe and validate the XML data.
The application layer of XML processing, such as a browser or editor, can use an XSL
style sheet to describe the presentation of the XML data.

4. XML data can be used for computer-to-computer messages, as well as for humanreadable
documents.
XML text and names may use any of the world's different alphabets, scripts, and
writing systems as defined in the ISO/IEC 10646 and Unicode 3.0 standards.

5. Internationalization (I18N)
XML text and names may use any of the world's different alphabets, scripts, and
writing systems as defined in the ISO/IEC 10646 and Unicode 3.0 standards.

6. Internationalization (I18N)
XML text and names may use any of the world's different alphabets, scripts, and
writing systems as defined in the ISO/IEC 10646 and Unicode 3.0 standards.

7. XML is a text format that is easily transmitted over the Internet and other
communications links. XML works with basic WWW protocols, including HTTP or
HTTPS.

8. XML will enable an enhanced WWW architecture. XML can also be used as a
universal data exchange and long-term storage format, with or without the Internet.
XML is constrained by design to be interoperable with SGML.
The Organization for the Advancement of Structured Information Standards (OASIS) is a non-
profit, international consortium that is devoted to accelerating the adoption of product-
independent vocabularies based upon public standards, including SGML, HTML, and XML.

9. Well-formed XML data conforms to the XML syntax specification, and includes no
references to external resources (unless a DTD is provided). It is comprised of
elements that form a hierarchical tree, with a single root node (the document element).
Valid XML data is well-formed, and it conforms to additional validity constraints of
the XML syntax specification and those defined in a DTD or schema.

10. SGML Note: The SGML equivalent of a well-formed document is known as tag-valid. The SGML
equivalent of a valid document is type-valid.

11. Parsers
There are two types of parsers:
Non-validating – the parser merely ensures that a data object is well-formed XML
Validating – the parser uses a DTD (or other type of schema) to ensure the validity of a
wellformed data object's form and content

12. DTDs
DTDs are an integral part of the XML 1.0 Recommendation, although they do not use XML syntax