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Extensible Markup Language (XML) is a standard for document markup endorsed by World Wide Web Consortium (W3C). It defines a generic syntax used to mark up data with simple, human-readable tags.

It was designed to provide a standard format for computer documents that are flexible enough to be customized for domains as diverse as web sites, electronic data interchange, vector graphics, genealogy, real estate listings, object serialization, remote procedure calls, voice mail systems, and others. XML defines the rules for encoding documents in a format that is both human-readable and machine-readable.

Although XML is a markup language, like HTML, it has no tags of its own. Rather, it allows the person writing the XML to create whatever tags they need, on the condition that they adhere to the rules of the XML specification. In a sense, XML is a markup language used to create other XML-based markup languages.

XML isn't limited to a particular set of markup. Users create their own markup to suit the data and document needs. The flexibility of XML has led to its widespread use for exchanging data in a multitude of forms.

Users can write their own programs that interact with and manipulate the data in XML documents, with access to a wide range of free libraries that can read and write XML, or they can use off-the-shelf software, such as web browsers and text editors, to work with XML documents.

Without a fixed set of tags and elements that are designed to work for everyone, XML allows developers and writers to invent whatever elements they need as they need them. Chemists can use elements that describe molecules, atoms, bonds, reactions, and so on, while real estate agents can use elements that describe apartments, rents, commissions, and other items needed for real estate.

Although XML doesn't include predefined tags, it does include very specific rules about the syntax of an XML document. While XML is flexible in the elements that it allows, it is strict in other respects. The XML specification defines a grammar for XML documents that dictate where tags may be placed, what they must look like, which element names are legal, how attributes are attached to elements, and so forth. This grammar is specific enough to allow the development of XML parsers that can read any XML document.

XML specifications define an XML document as a well-formed document when it satisfies a list of syntax rules provided in that specification. Documents that contain violations of the rules for a well-formed document are simply not considered to be XML. An XML processor will cease normal processing when encountering such violations. In addition to being well-formed, an XML document may be valid. A valid document is one that contains a reference to a Document Type Definition (DTD), and whose elements and attributes are declared in that DTD, following the grammatical rules for them as specified by the DTD. XML documents are classified as validating or non-validating according to whether or not they check XML documents for validity. A processor that detects a validity error must be able to report it, although it may continue normal processing.

A DTD is a type of schema, or grammar. Schema languages usually constrain the set of elements that may be used in a document, which attributes may be applied to them, the order in which they may appear, and the allowable parent-child relationships. The oldest XML schema language is the DTD, inherited from SGML. A newer schema language, which the W3C has named the successor of DTDs, is XML Schema, often referred to as XSD.

The purpose of this guide is to recommend online resources for the XML markup language.



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