Most pages on the World Wide Web are written in computer languages (such as HTML) that allow Web authors to structure text, add multimedia content, and specify what appearance, or style, the result should have.
As for every language, these have their own grammar, vocabulary and syntax, and every document written with these computer languages are supposed to follow these rules. The (X)HTML languages, for all versions up to XHTML 1.1, are using machine-readable grammars called DTDs, a mechanism inherited from SGML.
However, Just as texts in a natural language can include spelling or grammar errors, documents using Markup languages may
(for various reasons) not be following these rules. The process of verifying whether a document actually follows the rules for the
language(s) it uses is called validation, and the tool used for that is a validator. A document that passes this process with success is
called valid.
Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format which is both
human-readable and machine-readable.
from Wikipedia In computing, Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents
in a format that is both human-readable and machine-readable. The W3C's XML 1.0 Specification and several other related
specifications—all of them free open standards—define XML.
The design goals of XML emphasize simplicity, generality, and usability across the Internet. It is a textual data
format with strong support via Unicode for different human languages. Although the design of XML focuses on documents, the
language is widely used for the representation of arbitrary data structures such as those used in web services.
Several schema systems exist to aid in the definition of XML-based languages, while programmers have developed many
application programming interfaces (APIs) to aid the processing of XML data.
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